i have a ploblem with activate the iphone 3gs. i bought my iphone 3gs from thailand and now i was in USA but i reset my i phone and i cannot activate it

You have to have an unlocked phone if you bought it in one country and plan to use it in another. Did you buy one factory unlocked, or did you have your carrier in Thailand unlock it for you?

That carrier needs to unlock it.

Your Thailand carrier does not have cell towers in the U.S. If you are in the U.S. now, you can only get reception on a U.S. carrier's network. And in order to get on a U.S. carrier's network, your Thailand carrier needs to unlock the iPhone.

You CANNOT use AT&T until your carrier in Thailand unlocks your phone.

Buy a new, unlocked phone. The carriers in Thailand do not offer unlocking.

http://support.apple.com/kb/HT1937

Go to an Apple Store. An unlocked iPhone 4 is $450. An unlocked iPhone 4S is $549. An unlocked iPhone 5 (if they have them in stock) starts at $649 for the 16GB model.

To add to what KiltedTim said, "unlocked" means that you can use the SIM card of any type-GSM supported carrier in your iPhone, as opposed to a locked phone which can only be used with the carrier you purchase it on.

Thanks for the clarification on that stevejobsfan.

I have a ploblem with activate the iphone 3gs. I reset my iphone and i can't activate it pleas help me, thaks.