I just did something similar for a local youth group. Like LaRue said, think first about what it is that you can provide (group classes, seminars, bringing in speakers, personal training, etc). Then identify why you are the person who can provide these services. What makes you uniquely qualified for the position? Think about what you would charge (what you want to make, what the club would take, etc)? Also think about any equipment or room requirements you would need.
Take a look at what type of services they already provide, who they promote themselves to, the area they’re located and how you can help them improve upon these or add to these services. Why are they interested in having you write them up a proposal? If you can answer that question, then you have your proposal. What need are they looking for you to fill?