Some games are green camo (which include solid clothes of that colour) vs. tan camo, others require specific camo for each side, some have one team that is supposed to wear civilian clothing only.
The More milsim events you plan on attending the larger you're camo closet will grow, I've got so many BDUs I'm actually not entirely sure which camos I own. For gear, it's not always necessary, but after a few games you'll want equipment to assist with carrying ammo/equipment.
For you're gun, get age verified, we can't dirrect you to any retailers till then. But you can get new guns now pretty cheap.