Hey, and welcome from a fellow newbie! I might be new here, but I am not new to forum RPing. Here's what I have found from looking around here.

1) I haven't done any nation RPs, but my assumption is that you could probably do any group of people that are autonomous, such as what you just mentioned.

2a) Personally, I would post all of the base information that one would need to decide if they would want to join. That includes a brief insight into your plans for story, your expectations of the players' responsibilities, etc. How in depth you go probably depends on the category. If you're in Advanced, people will expect more from their DM. 

2b) Typically, you would announce interest, ask any questions you have, etc. I would do that first, then begin considering your character. The sooner a DM gets an idea of how many are interested, the better they can plan.

3) This is largely up to the DM. Check the OOC or Interest Check thread for their expectations. Generally, I have seen people expect a post or two per week, but it varies quite a bit.

4) Personally, I would always contact the GM, either through PM or the OOC, and wait until you get a response. Some RPs are marked to jump right in, but even with those, I would probably check with the GM.

Keep in mind that everything I've said can vary depending on the GM. I would suggest glancing through the rules in any place you plan to post and make sure everything lines up. That said, this place is designed to not be super strict, so as long as you respect the structure of the forum, you probably won't have any problems.