IPhone 3GS carrier switching

I recently got an iPhone 3GS from my cousin for free, but its locked to AT&T. I want to switch my carrier to tmobile somehow, because I have a SIM card that is fully working with my Samsung phone. Is there anyway I could change my carrier to tmobile? Thanks!

Added (1). EDIT: I want my iPhone 3GS to work with tmobile I meant

I can unlock for $20.
you can probably find a cheaper unlock on craigslist near you.

I will help you.
Before switch your carrier, you will need to unlock your AT&T locked iPhone 3GS. There are lot of ways available to unlock your iPhone. I would suggest Remote unlocking method. If you avail Remote unlocking service, you need not to jailbreak your iPhone. If you are interested in this method visit http://www.unlock-zone.com/apple-iphone-3gs/rs16wp9/ Recently i unlocked my iPhone using this way. I never had any problem. So only i would recommend this one.

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