First guy had the right idea, i’m deployed here and i design most of the t-shirts to be made in my facility, they normally higher a contractor who uses my images i make for the shirt and imprints them on plain colored t-shirts. They do it digitally (depends on the contractor and their….’skills’) and imprint it through either ironing (the cheaper contractors) or digitally, like a printer almost (first guy explained it better.).

Of course you can always paint or draw freehand, but the wear wash, and tear will most definitely put it’s toll on the t-shirt.

Cotton will most definitely shrink after a few washes, polyester or a mix of poly and cotton t-shirts normally work best.

for photos/images use a ink jet printer and computer and print using a teeshirt printing paper most art type shops sell them then iron on the design
for paiting any material is ok but cottons re best
i use Dylon pots here in the Uk whether theyre avaible where you are i dont know but there is probably something similar.
and i either trace on to the fabric or draw free hand then paint hope this helps