your client is already motivated to the point that s/he has approached you for personal training. While it may have been at the recommendation of a doctor, there is recognition of the need to change something.
People who have never exercised before in a formalized setting are often intimidated and may think that there is nothing they can do or do right. Since you have assessed you have a starting point. I found it always helpful to give clients a sense of success in every session I have with them. Particularly at the early stages, I only ask them to do something when I am rather certain that they can do it. As they gain confidence, you can challenge them at times with things that are a bit above their heads – but they have to be within their grasp.
It is also helpful to point out to clients things like “remember when you first tried this, and how hard it was for you?” People forget, and this can be very motivating for them to look back and see how far they have already covered.
Wish you good luck.