First of all get the t-shirt that you want to paint. It should be a plain t-shirt and should have a light colour to get best results. Then choose the colors of the fabric paint accordingly. You can choose either spray paints or liquid paints to be applied with brushes.
Carefully trace the design of your choice with a pencil. If you are using spray paints then draw the design on a cardboard and make a stencil.
Insert a card paper or some other paper like newspaper inside the t-shirt to prevent the back side from getting stains of paint.