>Was wondering where do I start on a forum like this?
Well, you can start at many places but I would suggest finding your roleplay comfort zone first. Find out which forum works best for you. There are seven different forums for a different kind of roleplay. The 1x1 forum is for, as is obvious, roleplaying with one other person. From what you have stated, it sounds like you would be good in Tabletop since it runs like a Larp does with campaigns and such. But as I said before, **you've** gotta find your comfort zone.
>Is it just one world or several worlds?
Oh, it is very much several worlds. Some are much like our own in their make while others might be different. It all depends on what the GM has made. And yes, some worlds you see through out our roleplays are designed by the GM. It is not rare to see a roleplay based around a premade world from an anime/game/book.
If you need some help getting started and figuring things out, the Articles & Guides section is a good place to start. It hold plenty of user created guides on how to improve your writing, how to make a good roleplay or how to make a good character. And remember, the number one rule of roleplaying is to be cool. Fonz cool.