How can I use my iPhone 3Gs on T-Mobile?

[rgelb]

Currently my iPhone 3Gs is on iOS 5.01. I'd like to use it on the T-Mobile network. What are the steps required to make it happen?
I think it's obvious that I need to jailbreak the phone. However, googling around around it is unclear whether there is a reliable (untethered) jailbreak for 5.0x.
Do I need to downgrade the OS? If so, to what version and how? Any additional steps I need to take?

 

[crowd pleaser]

If you are the registered owner, apply to the carrier, (who the only ones who can apply go Apple to unlock it).

Search jailbreak in here but the above is the. best method.