When choosing t shirts for screen printing you need to be aware of what the shirt is made of. You need a shirt with a high percentage of cotton. The screen printing inks will bind with the cotton fibers, but not the polyester fibers.
You also want to buy quality t shirts. If you buy name brands, you are probably going to be okay. Buy cheap imports and you will probably regret it. I buy my shirts at Michaels, Joann Fabrics or AC Moore. One of them always seems to have them on sale for cheap. But then I print as a hobby so buying small quantities is fine for me. I have found different styles of shirts at these stores. Regular t-shirts. Tagless t-shirts which tend to be a little expensive and also t-shirts specifically for women. Any of these will work for screen printing. Locally, I found Michaels tends to have to poorest selection, and AC Moore the best.
If you are printing any more than a small handful of shirts, Michaels, Joann Fabrics and AC Moore will not work for you as the quantities of shirts just wont be available, and then you will be paying too much for them. You will need to find a supplier that will sell you the shirts in bulk.
Color is another thing to consider. You need to think about how the color of the ink will go with the color of the shirt. But that really gets into the artist side, and is your choice.