Why does T-Mobile wants to take my money but does not want to provide the service?

It has been three weeks since I was supposed to have internet. However, T-Mobile does not provide that, although I have complained several times.
I argued that I would like to cancel my contract, so that I can subscribe to another company. T-Mobile says that, if I cancel it, I would have to pay for a year of subscription. Although the company has never provided me with internet in first place, can T-Mobile charge me? Besides, I depend on the internet to work!
What should I do?