Never tried something like this before, always wanted to. Sign me up to be a New Soviet Man, but can I get a run-down of how things work?
Given that this is your first time, I don't think playing as the Soviet Union is a good idea, and I think it would be best if a more experienced player were to control them. Basically, though, you choose a country and then roleplay as their government, taking actions accordingly. As explained in another thread, you are in essence the captain of the ship of state - but you are not the ship itself, and you are still subject to civil unrest, riots, revolutions, elections, coups, and the like, and if you're not playing a totalitarian state you cannot simply command individuals or corporations.