There are three "levels" of roleplaying here: Free, Casual, and Advanced. The general guidelines for the quality of post expected in each is given beneath the link to those subforums on the main page. There are also subforums for roleplays of specific types - e.g. 1x1, when two people want to roleplay instead of a group, or Arena, which is centred around battling rather than story etc.

To get involved in an RP (or to make one yourself!) you need to search the "Interest Checks" in each subforum. When someone has an idea for a roleplay, they can gauge others' interest in participating by making an Interest Check, describing the idea. Those who are interested post in the Interest Check, stating so. At this stage, you haven't got as much of an obligation to continue through and participate in the roleplay, even if you state an interest (although you really should.)
If enough people say they'd be interested, the GM (the person running the roleplay) makes a thread in the main subforum. Within this thread, there are two sections, or tabs - IC (In Character, where the roleplaying posts go once the roleplay begins) and OOC (Out Of Character). The OOC is where you sign up to be part of the roleplay, and the OP will generally contain much more information on the idea for the roleplay introduced in the Interest Check. Once you sign up in the OOC, you are committed to the roleplay - signing up then never participating is a big faux-pas, and if frequent and widespread often leads to the deaths of individual RPs and entire forums. 
Once enough people are signed up, the GM makes the first In-Character post, and the roleplay begins!

Hope that helps - and welcome to the Guild!