T-mineral cannot be mined on your base. There are two ways to obtain it, depending on whether you completed the "Crater-17" mission (#22 on your global map) or not.
Before "Crater-17": you can only get T-mineral by attacking other players' bases.
After "Crater-17": T-mineral deposits will appear on the global map the same way as gas and uranium deposits, and you will be able to raid the T-mineral convoys. Attacks on other players will still bring you T-mineral too.