Apple iPhone 3GS

Is there a way to use the 3gs iphone with straight talk which is 45 dollars or everything which is run through walmart. If so how would you go about it do you need some sort of hardware ie sim card and from who? at what cost? A software update or fix to change the carrier from at&t to straight talk? Will the speed go down because of it being a different network or is it the same type of network? Thank you.