Cingular, Sprint and T-Mobile are all good and have huge networks. If you're looking at somebody smaller, check them out first. Avoid Joe's Cellular, and the like, no matter how cheap it may seem. Stick with the big guys and assume you're coverage will be good, because it invariably is.Second, choose it by the overall price, and the freebies you get. You should get a brand new (not refurbished) middle-of-the-line phone (not top, too expensive), loaded with all the features you want. It should have a camera, blue-tooth compatibility and maybe some other features you want.
Don't be afraid of a one or two year service plan, assuming you want and can afford the phone. So long as it's an honest, well-serving and ethical company (like the biggies I named before), it's a fair investment. You know you need a phone, the coverage is competitive, and the rates are what you want.
And if you decide to cut your account short, don't pay the hefty disconnect fee (usually around $300), you can instead drop your plan to a $10 or $20 per month plan and throw it in the glove box as an emergency phone for good measure