It doesn't take much to paint hair. The greatest challenge you face will likely entail choosing the correct color. When you're doing this, experiment with the different hues to get the most realistic hair color possible.
Lay out the appropriate colors for the subject's hair on your palette. You will probably need to do some mixing with a palette knife to match the colors correctly. Most hair will require earth tones: blacks, browns, rusts, ochres and golds. Do not mix the color completely--allow there to be variants in the paint to give it depth and complexity.
Paint straight hair with a flat brush, and textured or curly hair with a round brush. For straight hair, apply the paint to the canvas in strokes (long strokes for long hair, short strokes for short hair). For curly and textured hair, apply the paint in lines that follow the shape and course of the hair.
For a shine on the hair, mix titanium white into the color. The amount of white you mix in with the paint will depend on the contrast of the shine on the subject's head.
Apply the area of shine to the hair where light strikes the head. Again, for straight hair, this involve using a flat brush and straight strokes. For curly hair, use a round brush to apply waves and curls to match the shape of the hair.
Mix a little bit of a slightly lighter color. With a detail brush, put smaller strokes of this color in the hair. This will diminish any "helmet head" effect, and give the appearance of individual hairs on the head.
You may need to adjust your technique depending on the paint you're using--acrylics or oils. Oil paint dries very slowly and makes blending easy. On the other hand, acrylic paint dries very quickly, requiring you to work quickly to get the same blending effects. If you are working with acrylic paint, you may also want to try acrylic retarder.
These suggestions do not address watercolors, which need to be handled differently.