Well first, is the phone unlocked? If it isn't, you won't be able to use it on any network other than the one it has been on. As long as it is all paid for, the prior carrier should have no problem unlocking it for you (as long as you were the owner).
Second, you will need to make sure that the phone is trying to access a carrier that uses GSM. Now from what I know, StraightTalk has both, some areas that are GSM and some that are CMDA (Because they access other carrier's networks), so it is going to make a difference. Don't bother with trying to set it up yourself, in this case. You would need to check with StraightTalk directly to see if your phone and the network where you are will work together.