As Jocelyn and Larue have so clearly articulated, just be absolutely careful about what you represent as your qualifications. I don’t know if you can get professional insurance, I doubt it, as LaRue suggests. And that’s a very important reality. Tami’s suggestion to “shadow” a trainer/client is great. What would you do in a similar situation?
I would at the first contact let every client know that you are studying to become a certified personal trainer. Bottom line.. it’s your skill and motivation that really matters. If you’re honest and up front, then many of your “pre-certification” clients will undoubtedly become clients when you complete the certification process.
Karin’s suggestions regarding assessment are so important. As a trainer, certified or not, assessment is the basis of your program design. Within the limits of the tools that you have available, become an expert at assessment of cardiovascular, strength, flexiblity and nutritional fitness. Then you will be an excellent personal trainer.