How do I set iphone 3gs apn settings for straight talk?

I have a Iphone 3gs from the company of AT&T and i switched to Straight Talk about a month ago and I have the app on the phone for straight talk but i can not send multimedia and can do everything else but not send pictures, group messages or videos nor can i receive them. If anyone by any chance knows how to fix this i would greatly appreciate it. Thanks in advance.