The good thing about fabric painting is that it is fairly fool-proof. The main "rules" I can think of are washing your shirts beforehand, not painting too thickly, and heatsetting.
Depending on your fabric paint, you can heat set either by ironing, throwing in the dryer, or even baking the shirts for short periods of time.
Ursonate is right--stencils are easy to make using any number of materials. But a Google search will bring up a ton of good sites with good stencils.
My favorite fabric paint is Deka (I know you can get it from Dharma (www.dharmatrading.com)...it comes in tons of colors, has no stiff feeling whatsoever, and you can iron to heatset--which I think is the easiest way. Actually Dharma has a ton of supplies and their catalog is very informative and can even give you some instruction (on painting and also on what products are the best for you). Have fun!