The site here is mainly focused around a single world (Valucre), though RPs in other universes (such as a thread set on Earth) are allowed in the 'Alternative' section.
The In-Character boards of Valucre are divided by setting. If you go to the main page and scroll down, you'll see that the IC boards are divided into three categories: Terrenus, Genesaris, and Tellus Mater. All the sub-boards within these categories represent the different regions of Valucre, and you'd make a thread in one if you wanted to roleplay there. For instance, if you wanted to do something within the city of Last Chance, you'd go to the Last Chance board and make/join a thread there. Alternatively, if you wanted to go exploring the seas near Genesaris, you could jump into the Islands board... essentially, every section is a setting for people to roleplay in.
Actions within a setting can change it, and even have larger effects on the world of Valucre itself. Several cities, for instance, are ruled by different players, who succeeded in taking them over and shaping them under their rule. Biazo Isle, meanwhile, used to be a prospering city- until a bunch of players blew it up, and turned it into a smouldering ruin. The rules on making your actions in an area 'canon' vary from place to place, but a general guide can be found here.
I believe you've already found the Tavern of Legend, which is a board for new people to test the waters. Other than that, some useful locations include:
- The Profile Database, where you make sheets to keep track of your characters. Having a sheet here will allow you to quickly link it to others to give them a general idea of who you're playing as. It's also useful for keeping track of your character's accomplishments, relationships, and other developments that occur to them through roleplay.
- The Water Cooler. This is the board where people organize and recruit for roleplays. If you're looking to get involved in something, this is the place to go.
- Open roleplays. Any thread tagged 'open' on this site is technically free for anyone to join, so feel free to hop in one if you see something you like. It's generally considered polite to let the thread starter know you're jumping in, but there's no obligation to if that tag is on the thread. Threads tagged 'closed' and 'semi-closed' (or not tagged at all) are more private, so if you want to join one of those you'll definitely want to ask beforehand.
- Free Chat. For random banter and stuff.
The Getting Started guide also covers a lot of what I've said, if you need a more formal perspective.
Hope that helps, and good luck!