What to do
First, make sure that you're familiar with the site. Read through the main site, as it has most of the information you'll need on when and where the site is set. A lot of members' questions on the setting of the site can be answered by reading through the pages on the site. Once you're familiar with what the main site has to offer, visit the forum and try reading through some threads there.
Start simple. If you're not sure where to take your character, of just how to begin on the site, first deal with the basics. OOC stuff first - sign up, introduce youself, read the rules, and submit your character application. Once you've done that, post in IC introduction, going over your character's immediate past, and how he or she got to where they are now. The best thing is just to get involved, and you'll feel right at home in no time.
Be polite. I know it's basic, but simple courtesy will solve a lot of problems. If you want to join in a thread, ask the people involved. If you want to roleplay, PM other members with some ideas, or post some in the Discussion forum. Leave threads open-ended if you want others to join in, and remember to communicate. Don't expect others to do all the work for you - sometimes you have to make your own fun - but don't be afraid to get involved. After all, that's what roleplaying is all about.
What not to do
These, too, are fairly basic. Above all, don't godmode or metagame. Godmoding may be defined as having a perfect character; metagaming, as using OOC knowledge to influence your roleplays.
Having your character be smarter, prettier, or stronger than everyone else may be tempting, but it's also boring. If an administrator thinks that your character is unreasonable, you will be asked to edit his or her biography - that is, if you can even be bothered playing such a perfect character in the first place.
Metagaming, likewise, will not be tolerated. You are always going to know more than your character, whether about others' thoughts and actions, future events, or simply knowledge of the world of the Wheel of Time. A good roleplayer is one who is able to separate IC and OOC knowledge.
Tips for roleplaying
Make your character interesting and unique - but not perfect. Not every Novice has to be an Andoran noble, or every Recruit a Borderlander trained in warfare. Prior experience means nothing once you begin your training, so, unless you chose such a background to highlight your character's actual ignorance and naivety, it will actually be more difficult to roleplay someone supposedly experienced in the sword as having to start all over again when he or she enters the garrison. Weaknesses are not only a great way to distnguish your character from others, but they are far more interesting to roleplay. Maybe your character is a former Cairhienin noble, ambitious and powerful in her home land, but who, upon reaching the White Tower, is weak in the Power, and can barely form complex weaves. You'll have more fun roleplaying her, and others will have much more fun reading about it.
Along the same lines, try to avoid cliches. Not all Novices and Recruits fall in love at first site at a festival - and those that do, will most likely be scrubbing the kitchen floors with their bare hands for a year. Not all women in the Wheel of Time are beautiful, or stubborn, or have fiery tempers (even if Robert Jordan does write them all that way). Try something different - if only to see how you like it.
Think about others. A little consideration goes a long way when you're roleplaying. If you're roleplaying with someone else, read through their posts carefully - don't just skim. A lot of times, their posts will contain information about how their character is acting, about the characters' surroundings, or hints about where they would like the roleplay to go. Don't move others' characters - even a little - without their permission, hijack threads, or make sweeping statements. You may be the first person to join a class - this doesn't mean that no-one else will join, or that the teacher hasn't decided that there are NPCs present, so don't say that you're the only one there. Moving others' characters in general is just as bad as moving one particular character. Ask where others would like the thread to go, leave something in your posts to work with, and use their posts to guide your own.
Description is your friend. It isn't hard to write a lot, or to write well. Posts should contain more than just action and dialogue - they should contain thoughts, feelings, and observations. First, think about your settings. What time is it? Season? Temperature? Where are you? What is going on around you? Second, think about your character; what are they thinking? Feeling? Where are they coming from, and where are they going to? Why? Third, as I've said, take your cues from others - how does your character observe others? Remember that your character's observations do not always have to be true, so have fun with what your character doesn't know.
And now for my favourite point - have a plan. This can be as general or specific as you want; all I want you to do, is to think. Think about what you've put in your biography, and why it's there. If your Novice grew up rich and beautiful - why? Is it so that her forced humility as a Novice is all the more poignant, or just because you wanted her to be rich and beautiful? Think about where you want your character to go, and how you want to get there. I get a lot of requests for special consideration - rare Talents, roleplaying opportunities, etc. - and most of the time, I'm more than happy to grant them. The one question you'll always be asked, however, is why - so give it some thought.