Thank you for volunteering to become part of the force that strives to keep 4chan one of the interesting corners of the internet. As a janitor, you will be one of the elements that help to shape each board.
This will helpfully guide you as you start with your new duties. First off, let's go over the abilities and restrictions you have now.

1. Remove comments/images on the assigned board
Deletion is your primary tool and concern. You can delete individual posts, threads and images. Deleting an image will leave the "File Deleted" placeholder, so take that into consideration, as it can be a sign of your activity to the users.

2. Submit ban requests to moderators
You can contact the mod team through IRC (more on that later) or file a ban request in the reports systems. Remember, this will be submitted to the mods consideration. It is akin to a suggestion and the mods will decide if banishment is the best course of action. You will not be able to ban, either directly or by proxy.

3. Access the reports queue to more easily identify offending content
The reports system is your best friend. It will show you what the users identify as rulebreaking or outright illegal in your board. We're all just human, so a lot of threads you would ignore in your normal browsing will show up here. We will go into more detail on this system later.

Some of the limitations you will find:
1. Enforcement, not interpretation
You will follow the rules and policies established on a board by the mods and admin. That means you can delete posts and threads that violate the rules, but you cannot establish or expand on these.

2. Silence
Janitors do not have capcodes and do not interact directly with the community. You will be anonymous and cannot disclose any of the details about staff, its tools and systems. Breaking this confidence means you will be retired.

3. Accountability
You will not be a rogue, or a loose cannon. Janitors are expected to keep in touch with the rest of the team and your actions can be held up to review. We're all in this together and you are now a part of a larger team.

We will expand on why these restrictions are important later on.

Now, let's go over what your goals are now. As a janitor, you are expected to be knowledgeable of your board and its users. You will use this as a tool to better steer the board into a place that is enjoyable for all. You are also encouraged to relay this information to the mods and the rest of the team, as you're now its representative. Your main task is to keep illegal content out of 4chan and the board healthy and with a decent amount of quality.


  On why silence is such an important aspect
  
As a silent enforcer, you follow the rules but don't get to clarify them to the userbase. This can, and will be, frustrating since the userbase can be confused and misread your actions. The reason is that you're but one of many. The goal is to present to the users a unified front of moderation. Consistent and constant. If you're identified as an individual person, you become just a man with opinions and emotions and they will prey on it. It's better to leave the moderation members to post publicly as they have experience on their side, along with the confidence that they're trusted on how best to approach the community. Anything said can be distorted and reposted for years causing quite an amount of damage, no matter how good the initial intentions were.

  Guidelines for deletion

Now here's a tricky one. What should be deleted? "Rulebreaking posts" would be the easy answer, but it's far more complex than that. There are off-topic threads that don't add to the board, but don't detract from it either. As long as they are not directly harming the board environment and providing some fun topics for the users to post in, there's no reason to delete them. This covers some topics that can be found in other boards besides your own but that due to userbase interest and culture can fit in your own. We can call this Topic Overlap and it covers things that could be argued to go in other boards but don't really cause any harm by leaving.
The most difficult rule to uphold is of course Global Six, which determines that some level of quality is required. This will not always be the case and "decent" is some times the best you can hope for. Try to watch too closely and by deleting anything that doesn't fit an arbitrary level of quality only creates a hostile environment that the users will soon no longer want to be a part of. This doesn't mean you have to allow shitposting, just that you have to ask yourself if you're not making a bigger impact by deleting it than by ignoring it and letting the users figure it out for themselves. There are a number of things that make the 4chan imageboards special among all of what the internet has to offer. One of them is the relaxed environment and atmosphere, take care to not stiffle discussion and target only the real threats: spam, shitposting, flooding, unrelated content that ends up in major shitstorms.
Remember, the most important thing is to let users have fun. Obey the spirit of the rules, not their strict letter. As long as the fun does not come at the cost of the board, then it should be okay.

  Board culture and why it must be protected
  
Let's start by defining board culture. It's the shared history and tradition that springs naturally from a board after years of threads, inside jokes and memorable events. This means that not everything gets a pass as culture and the general rule of thumb is that if it's enjoyed by a significant portion of the board while not being forced or spam, then it's legitimate culture and a fundamental aspect of what makes every board unique. After all, each individual board is defined not only by the topic in the top of the page, but by the unique userbase and mentality you find there. Roughly, this means that what would be considered worthless shitposting in one board might be part of the cherished culture of another. Where you draw the line will depend significantly on how much you know your board and how attuned you're to it. 

  Janitors as a conduit between users and mods
  
As a janitor, you're now a step above the users. This is not a power step however, but a information flow. You're now a representative of your whole board and you're meant to rely to the rest of the team, the mods and the admin the happenings and ongoing events of your board. This way information flows up the chain and allows everyone to be informed of what happens in all of 4chan without having to lurk every single one for every waking hour of the day. If your users are worried about something, like a notorious ban evader or a massive event that could benefit from a sticky, try to relay this information to the channel so that mods can make an informed decision.

  Cheese Pizza

Your biggest priority is and always will be to strike illegal content, specially Child Pornography as quickly as possible. Do not wait for confirmation, do not ignore the thread. Just delete and request a ban through the system as soon as possible. The ideal is to minimize the time such content is present in 4chan. Please make sure of making the correct request, as child models, nudity without sexual context and outright porn are different categories. Mods are not able to see the images themselves when filing a ban for an already deleted post, so you have to take care of using the correct description to make a meaningful ban message.

  Users are tsundere at best
  
Users will be happy and praise you, while spitting on you and calling for your dismissal in the next post. This is a normal part of dealing with a large, anonymous userbase since it's difficult to know how many people are taking the effort to post such feedback. Know this, you cannot keep every single user happy, so aim at keeping the whole board to a decent level. Review and consider objective criticism so you can improve yourself and your work, but do not let this freeze you. Treasure the posts that commend your work and efforts, learn from the ones that question a particular action, and ignore those that just baselessly call you a faggot for getting in the way of their own selfish fun.

  Sisyphus and his rock
  
You will push a rock uphill every day. You will go to sleep and wake up to find that rock has slid all the way down once more. You should make your peace with this reality now, for we are but men and no single janitor can delete absolutely everything every hour of the day. You might wonder why so many continue to indulge in such punishment, and the short answer is that it's an enjoyable task. You know have a hand in making your board better. People will appreciate your efforts and note when you're missing. But most of all, you get to fight the decay head on and help 4chan remain fun for more years.
It will be repetitive and exhausting. It's up to you to find the reward every time you manage to clear that hill.

  ...but there's an I in PIE
  
http://www.youtube.com/watch?v=sgxoM37SQs0 You're now part of a select team of janitors. There's no such thing as seniority or rank, we are all in this mess together and more often than not, your fellow colleagues will be the only support you'll find to endure the endless task you have chosen to take on. The IRC channel is more or less a band of brothers that have come together by their shared love for their boards and 4chan, with an accumulated knowledge and experience you can always feel free to rely on. Discuss policies and how best to respond to some threat. Coordinate with the mods so you don't end up deleting something they're preparing a ban for. Share what users are being a pain or something fun you found in your board. Or just talk about a recent anime episode or football match. Nobody else in the world knows what you're going through but these people, become a proper team so you can avoid getting burned out.
The channel is a place to coordinate, discuss the boards and take it easy. Never go rogue and delete without at least lurking there so everyone can be aware of the current situation.

In conclusion, you are taking one of the hardest jobs possible in front of a computer, for no money or tangible reward. However, this can be very satisfying work as an essential part of what makes the 4chan run smoothly and remain one of the best sites on the internet for discussion or just dicking around.
Thank you for choosing to be a janitor, best of luck and have fun.