How much shampoo should I use?

About one pump is about right for medium length hair. Professional shampoo is very concentrated so only a small amount is necessary. Generally speaking:

Wet your hair thoroughly Apply the shampoo to your hair Lather Rinse thoroughly

But, as always, read the instructions on your particular shampoo to ensure that you are using it correctly! If you are looking for a change consider trying TIGI's Eco Awesome from their Love Peace and the Planet range. It's organic.