Firstly, welcome to the Guild 2.0! xD

On one hand, I like that you actually have questions but on the other hand, I think you might be psyching yourself out a little. 

1.) [url=http://www.roleplayerguild.com/topics/4958/posts/ooc]User's Guide to the Guild[/url] should help with clearing up a bit.

2.) Each role play is different; some GM's decide the post-speed and some just let it go freely.

3.) Either way, it doesn't matter. I would personally recommend starting with a clean-slate RP that hasn't already begun, but it is truly up to you.

4.) I was around 11 when I started roleplaying (god, don't ask - my mom didn't monitor our computer time) and I can't remember much of it, started in Yahoo! Chat. All I can say is...start small. Don't push yourself into doing huge posts with a bunch of filler crap if you're new to roleplaying. You can work your way up like most of us did.

5.) Roleplaying is serious business, buster xD.