Gary, you'll have to do some research to help find which phone (make that phone company) is best for you.
Ask friends and neighbors in your area who they use and how their coverage is.
For a first time buyer, you may want to steer clear of ebay for your phone, go with a place you can walk into if you have a issue with the phone or your service. Again, who you choose should largely revolve around who has the best coverage for you in your area. All companies are not equal in coverage, so be sure you do your research on that point. Buying off of ebay does have one main advantage tho... It allows you to buy a decent used phone WITHOUT locking you into a 1 or 2 year contract. That is how and why I bought my current Android phone to use on Verizon's network... No contract, so I can bail or upgrade when I want without facing any penalties or limitations.
Locked vs unlocked is largely a issue associated with GSM technology phones that utilize a SIM card (ATT). Verizon (CDMA technology) does not use that protocol except in their "world phones", and their two networks do not play with each other at all. So, if that new tower belongs to ATT, buying a Verizon phone may be useless to you... And vice-versa. If you have a phone with a SIM card (say ATT), you can remove the SIM card and insert it into a different unlocked GSM phone from another network (say T-mobile) and then start using that phone on your network with your same phone number. Contacts and other info can be stored on the SIM card as well so they transfer from phone to phone with the SIM card.
While some companies offer a so-called "$15" plan, be careful with them. They offer those prices because they either have very limited coverage, or there are other fees associated with it's use. They are usually associated with pay-per-use phones which usually cost you about twice as much.
I hope that helps answer some of your questions.