Are you talking about client notes, like how much stronger they’re getting and at what exercise?
I use ptonthenet, which I can access from my home computer, from the internet at work, and on my iPad with wireless. That keeps my individual client workout data, so I could send a client a workout from anywhere, and I could look at their last written workout to see weight / sets / reps.
I would start with a really great client management software. I like www .fitify.me make sure you use one that will let you post your schedule so you can run your classes in multiple locations. Fitify lets you sell your session to your clients in block and the system remembers and redeems them as they are used. Clients can also book into your schedule from any device, it alerts you to the requested time and bills the clients account. Also having the ability to collect multiple forms of payment will help branding you into a legitimate business. They let Make sure you can organize and contact all those people that took your classes in the past. They you can do things like a message blast out to all your clients saying you have an availability. I like Fitify cause they are really good to the little guy, they don’t cost much but they really bring legitimacy to any one person business like a personal trainer or palates, yoga & zumba instructor.