Welcome to Iwaku!
As for where to start, we try to make it easy and understandable :)
First, in OOCS, look at the title next to each thread. We use a classification system for the content of the roleplay (so modern will be semi-realistic, taking place in modern day, etc.).
Choose one that seems interesting and read the first post where the story is generally discussed. If you don't think it's a good fit, rinse and repeat the other instructions. If you do, then look at the character sheet they have listed (unless it's a jump-in, but most of those do not have OOCs).
Post a character and wait for the person to approve of it.
Once that's done, go the the section corresponding to the title from before (i.e. Modern board for the modern OOC thread).
Post in character!
If you need any help, please feel free to ask me. I'm Kitti, by the by. :3