Which company? It really depends on where you live. The first thing to do is ask around, and see what service your friends and acquaintances have, and how they like it for signal quality and problem resolution when they have to call customer service.
When you figure out a couple of candidates, shop around and find out what kind of deals you can get. I'd recommend going into the provider-owned stores, since if a problem arises or you're misinformed about something, you're just dealing with one company. If you do go with an independent retailer, I'd stick to stores you know that have decent return policies and good customer service after the sale.
Ask lots of questions. Find out what your plan includes and what you're charged extra for: overage, roaming (on-network and off-network), long distance, directory assistance, ect. Find out what the free cancellation period is - this is the time frame (usually 14 - 30 days) in which you can cancel the service and not pay a fee for terminating the contract with the service provider. If you go with an independent retailer, find out if they have their own contract with you, and if so what the terms are.
Whichever way you go, after you've made your purchase, I'd call customer service and confirm everything your rate plan has, just to double-check what the sales person tells you. That way you won't have any suprises on your first bill.
Generically, I'd recommend either T-Mobile or Verizon. Both generally have good customer service and good coverage where they offer service. I wouldn't recommend AT&T, and I'll tell you to stay the hell away from Sprint, as their customer service is ass. But really, check with friends and family locally to see who they use and how they like them.