Before choosing a prepaid company, have an understanding about how much voice, text and data you use so you can dial it in and not spend too much.
I am familiar with Tracfone, NET10 and Straightalk. All 3 are owned by the same company and each is geared toward a different level of user.
Tracfone: Low to medium use. Primarily for voice and text. May not be a good choice if you are heavy into data use.
Net10: Medium use. Has an all you can use plan for $40.
Straighttalk: Power user. Best phones of the 3. Iphones and latest Androids also.
I have used Net10 and now moved to Tracfone. Very happy with both. They use both the ATT and Verizon networks depending on which phone you buy so coverage and call quality are good.
If you do not use a lot of data give Tracfone a serious look. It is cheap and they have android phones for about $120. Although they are 1 release behind generally.
I believe all 3 companies have a bring your own phone option. Meaning they will send out a sim card for your phone to work on their network. I am not sure how the nuts and bolts work on this feature, you will have to search on it to know. Also, I am pretty sure Net10 and ST may have a limit on how much data you can use, so check it out to make sure that would be acceptable to you.
Howard Forums is a pretty good site for cell phone info and have a section dedicated to prepaid phones.
One last thing, if you do use Tracfone etc, get a copy of the Opera mini browser for your phone and stop using the browser that came with the phone. The reason is you will be billed a lot less minutes when you hit the web as the default browser is connected from start to finish and the Opera mini only connects sporadically.