I recommend compression tank tops rather than t-shirts, worked better for me, little bit more breathable under the arms.
I was wearing a compression tank for nearly a full year before I went full time and got through all seasons with it.
Barely Breezies is the brand I used to get, they have a few different firmness options and are priced pretty cheap in comparison to other brands.
Also be careful of getting the really tight compression tops that are similar in strength to binding, they are pretty common for FtM's, the reason I say be careful is because they can be way too tight for developing breast buds, you gotta take care of them, not squashing them too much, especially not in the early stages - they are fragile this early on! plus it hurts like all hell.
Jay-Bird