you can get a sim card from any bmobile or digicel outlet/dealer. There are plenty around and all you have to do is walk with is some form of photo ID and $50 TT.
your android phone should work fine here. Tmobile uses GSM, so no issues. We do not have 3G cell data in T&T.
You might need to call Tmobile and request they unlock your phone. If you are out of contract, or bought your phone at full retail (no contract), they will more than likely unlock it for you to use in T&T. I know someone did this personally and it was no hassle; currently they are using bmobile on a Tmob Galaxy S, no issue.