As a T-shirt artist I will agree with the Createx that 1337 mentioned.
It and Aqua Flow are the best textile paints out there.
Aqua Flow's opaque white is actually better than Createx's but it is harder to find. If you have a Michael's near you or another art supply then go buy a bottle of Createx opaque white and use that.
If you want the paint to last a long time it needs to be heat set after it has dried to the touch. The only way to get it permanent is to use a heat press but they are expensive so you will have to resort to using a hand iron.
Put a protective sheet such a Teflon sheet or even a piece of wax paper between the shirt and the iron and Iron the shirt in a slow manner covering the entire design.
Make sure you never wash the shirt in the washer except inside out and on a gentle cycle. NEVER put it in the dryer after it is heat set.....EVER!
Air dry it and it will last a long time without fading.
Here is what Createx's web site says:
Fabric- Including 100% cotton, 50/50 blends, denim, sweatshirts, and most natural fibers.
1) For best results, wash item first to remove sizing and mill finishes.
2) Airbrush the design onto fabric avoiding thick buildup of paint and excessive paint layering.
3) Allow colors to dry, or a heat gun may be used to accelerate drying time. Drying time varies depending on application.
4) Heatset fabric for permanence by one of the following methods:
a) Iron for 2 minutes at 300ยบ F use a protective cloth, keep iron moving.
b) Turn item inside out and put in clothes dryer for 40 minutes at highest
heat.
Mike