Well you have to gain alot of pl first then when you can (your CF bar which is above the pl bar, is fully green) you hit the z button to transform. For any reason that you want to cancel it or go back to normal state you hit the x button.
Remeber, it taks a little bit of ki to transform and if your trasnformed with a certain character and your ki depletes, you will go back to your normal form.
Each character has a certain Power Level to trasnform at, for example, trunks gets to go super saiyan at 1,000,000, while goku has to wait until he gets 1,750,000. This is all explained in the link above and more.