Your exercise library is the tool that helps you build the workouts that you'll send to your clients, and that's why you need to build it before you can begin working with clients.
For now, your library doesn't need to be extensive. Make it simple so you can begin working with the product!
Consider starting with 5 exercise videos that could be assembled into one workout so you can move right into the process of recording audio cues and building your first workout.
Once you understand how to build a workout, you can go back and add more exercises to your library.