You'll need to get copies of birth certificates, purchase the insurance, figure out which tournaments to register for, figure out when/where you're practicing, order uniforms, etc. And, above all, clearly communicate with the parents on expectations, scheduling, competitiveness, playing time, roster management, money, etc.
USSSA runs better tournaments than Nations, IMHO. But Nations isn't a bad place to start because the competition is weaker.
My son is on a 9AAA team in USSSA. Most of the boys have been together since 7AAC. This fall/offseason the coaches bungled some roster management and parent communication. A formerly tight-knit, fun group has turned into an awkward, not-as-fun group. Be careful with that dynamic. Hard decisions that hurt feelings are one thing. Unnecessary decisions that aren't communicated and then hurt feelings are another thing, and much harder to get over.
Select ball can be fun. Our particular group usually doesn't play during the local league season. We pack 4 or 5 tournaments into June and that's it.