Hello,
First of all, "I HAVE READ THIS". Secondly, yes, I actually read your description and I think your project is pretty straightforward.
I can adjust my bid amount depending on your project requirements and your budget.
There will be many tables in your database, for example "customer", "option", "payments" etc. It will be relational database. As a start, Microsoft Access Database can be good option for the back-end database.
May I suggest to build your front-end of application in C# or VB.NET using MVC (Modal-View-Controller) framework. That way, in future, if you decide that you need to move your data to MSSQL, MySQL, or PostgreSQL etc. then you'll only need to update "modal" part of application and your front-end will continue to work as-is.
I'll also suggest to have a "dashboard" at startup of your application. A quick glance of info instead of digging deep. For example, on your dashboard:
a) How many customers you have
b) Number of customers (quick info) whose membership is going to expire
c) Which part of your gym is most (signed up) occupied (gym, mma, boxing etc.). It will help you to either add/upgrade equipment or create marketing promotions for neglected areas
d) Many other kind of information that you would like to at your quick glance
e) GUI should be touch screen friendly for your future upgrade
I have many more suggestions but I am running out of space. Please feel free to message me. Do check my profile.
Have a nice day. :)
Best regards,
Farhan