How to unlock iPhone 4 for tmobile carrier?

Like unlock the SIM card so I can use tmobile my mom went to tmobile but they said I have to do it by mail if you did this how did you do it or is there a place I can go to

Jail break it first then google how to unlock it both are very easy to do trust me

You can jailbreak (very simple to do if on iOS 5.1.1) or pay a 3rd Party to unlock it (mall or online, just look up unlock iPhone 4 and look for some or go to the mall and see some 3rd Party phone stores)

You can actually take the phone to your local T- mobile store and have them do it. They call someone else to activate the sim card and unlock the Iphone for use. After they finish that, the sim card can be placed in the phone and it can be used like any other purchased phone from the store or any other company

As others have said, you must jailbreak first in order to unlock. You can jailbreak with Absinthe and unlock with ultrasn0w (there are other options as well).

If you are not experienced with modding phones, I would spend some time researching the process. A good source is the iPhone Wiki (easy to find via google). Make sure to do a full backup before trying anything, and be prepared to spend some time troubleshooting during the process. Also, you must be very careful when jailbreaking, making sure to follow instructions closely. Carelessness can result in a bricked (broken) phone.

Alternatively, you can hire someone to do the jailbreak/unlock for you. There are a lot of people who do this, so competition tends to keep the price low. This is a good approach if you don't want to spend a lot of time learning and fiddling with your phone.

Tmobile is getting the iphone in 2013 in case you are wondering.

Depending on which carrier the iPhone is with now, there are services that will factory unlock it like iPhoneUnlockedCheap.com if it is currently with AT&T. They do it in less than a day and for cheap prices too.