You can call TMobile and they are usually able to unlock your device over the phone through their device unlock application if you are unable to go to a Tmobile store. There are no negatives to unlocking your device- you are able to use the device with other carriers as well as use it outside of the U.S with other sim cards.
Hope This Helps!
To give some follow up information on what @ilanag96 provided; here are some related links.
Also just keep in mind the only way T-Mobile typically unlocks your device is if you do not owe anything on it (bought out right or paid off the payment plan) or you ar traveling outiside the U.S. (it may only be a temp unlock if you still owe money) or you are in the military being sent out overseas.