I got some advice for yo RP. One thing is something you should never do. NEVER EVER. That is running character/group RP and nation RP together. The nation RPer would have several advantages over a character RPer. So, your RP won't go well if they start to clash together. I had this mistake back in my RP. Ragnarok had that same mistake, which lead to their downfall. So, in conclusion, focus on one type of RP only.
One other thing, try to be as liberal as possible. I meant was don't be a dick with creating stupid ass rules or systems such as having an economy system with sets of rules that go with it. Lets be real, no one wants to do math in a entertainment roleplay, unless your contestants are a bunch of motivated ass accountants, then that would be a different story. Try to have as many voting polls as possible. Don't overdo it however. As I said, there are some things you should never do, some depending on your skill. For example, lets say you are doing a fantasy RP and someone wants to RP as dragons. Dragons are more likely to be overpowered as shit and have a higher chance of powergaming. So, if you can limit that by lets say having less dragons or modified health, by all means go ahead. But if you can't handle this, you shouldn't try to have that person play as a dragon nation. Other than that, good luck on your first time as DM.