Try following the link Viddyvane posted. If it still doesn't show your phone as compatible, then 1 of 2 things could be at play.
1. You may have to get the phone unlocked from Sprint, or
2. Your phone may just not be compatible with Straight Talk.
You can try using the Sprint Straight Talk Sim card in your phone, if it works, then no worries, if not, you may have to get the phone unlocked as I mentioned. When in doubt, try contacting Straight Talk?