You can't strengthen a signal in a receiver (which is what I assume you mean - that you're receiving a weak signal). You can use a better antenna or you can redesign the receiver (neither of which you can do in a cellphone). If you're in an area where TMobile has weak signal, that's where you are, and you shouldn't be using them as your carrier. The first choice is always the carrier, and that's always based on which carrier(s) has a good signal where you need a signal.
It's possible that the antenna in your phone isn't making good contact, but that's not something you'd repair yourself, unless you have the experience and test equipment needed. If the signal is weak all over, bring the phone into TMobile, or a good repair shop, and have them take a look at it. If the signal is weak only in some places you need a good signal ... well, as I said, TMobile shouldn't be your carrier.