Make sure your contacts are in Google contacts, not on your SIM card. Photos should be backed up to a cloud service,like Google Photos. Call T Mobile to make sure they have "unlocked" your phone, meaning you are free to use another company's network (your phone has to be fully paid for and belong to you; otherwise they keep it locked). Then all you have to do is buy a SIM card from Straight Talk, and make sure they have the phone number. Then you pop the T Mobile SiM card out of the phone and replace it with the Straight talk SIM card. Your apps, photos, etc should not be affected, provided they were somewhere other than the SIM card.