First off, hello and welcome to the Dantooine Theatre Company (And the forums) Now for your answers:
1. Depends. Find a thread with a good compelling story that interests YOU and that you can contribute well to its story.
2. This depends on the situation. Generally, your character will speak in first person and your character's thoughts should be written in third person. However, some thoughts can be presented in first person, depending on what type of effect you want them to have. Just read a few posts to see how it is done. There are a few ways to do it but they all follow the same basic rules. Just look around and you will get the hang of it and develop your style. It is very much like writing a work of fiction in literature. Also, as a side note, you as a person should rarely, if ever, refer to your own character in third person. You can however, and this is more common, speak about a situation or another person's character as yourself and not as your character. If you do either of these, however, be sure to mark it OOC (Out of Character) so people know that it is you speaking and not your character. I would also suggest reading the stickied "Read Me!" post that Raven did as well.
Hope that helped some. As you see, there are not just simple "yes or no" answers to the questions.