Using iphone 3gs with straight talk?

Okay I'm confused. I have a straight talk smart phone already but want an iphone so i bought an unlocked 3gs to use. I put my sim card in it thinking that would be all i had to do but it won't work. I can text on it but when i try to use the internet it says it isn't connected to the internet. Do i have to buy a brand new sim card and activate it then? Also i can't get hardly any service, one bar at the most when i would normally have full service. Ugh help!

You need to change the phone's APN settings to match what Straight Talk has on it's website. The APN settings can be found under the "settings" menu,