Can I use my Sprint iPhone 4s on straight talk?

Part of the way through my father-in-law's 2 year contract with Sprint, his phone stopped working, so he paid full price for an iPhone 4S. After the contract was finished, he diclined to extend it and went on to buy a different straight talk phone and sold me his iPhone 4S and worte out a bill of sale for me. My question is, now that I own the iPhone 4S, can I use it on straight talk or verizon pre-paid? I have looked online for answers to this question and have heard every variation of every answer. I know that it should be able to be done, I just don't know which company is best to switch to, and if I'll need to go through some process of getting it "unlcoked" so it can be used on another carrier other than sprint. If anyone knows the answer to this question, please let me know. So to re-cap:

- I own a iPhone 4S that was bought at full retail price and used on a Sprint contract.

- The contract is now up and I want to use it on a pre-paid service such as straight talk, t-mobile, or verizon "pre-paid".

- Which sevice is the best to use without a contract?

- What is the process of switching the phone over the carrier?

- Do I need to "unlock" the phone from Sprint?

- How do I "unlock" the phone if that is what has to be done?