If you are a member of IDEA you can go to the page for Personal Trainer & Fitness Professional Forms and download one.
With which organization did you certify? I ask because sometimes a certifying agency may have standard forms you can use.
Generally it is better to speak with a lawyer to help you develop a form like this, as well as to give you guidance you will need regarding local and state regulations with which you will need to comply.
I have two different forms for the different formats. For group classes, I only have people sign a liability waiver. For personal training, such a form is part of my contract package.