You need to plan and figure out how are you going to implement things. You can use different tools like ArgoUML etc. for diagrams.
Different things you can include in your project:
-For the main interface: Addition of new members, deletion of members, update in the existing info of the members, stock available of equipment, information of issued equipment, event dates, coaches details etc.
-For a member: Joining date, membership expiration date, gender, payment status, team, joined sports etc.
You can use class objects to perform read/write operations with the files.