Welcome to Mendix.
First check out this video recording of the creation of the habit tracker a couple of months ago: https://video.mendix.com/watch/x2GSFXnPgYq98P2r6f2wNG
And the download of the source code: https://bit.ly/3gVDF3U
Using that as your base you will likely be able to get what you need.
Whenever I want to create a new app I simply write down all the activities that need to be done on paper. After that just create a rough view of your app on paper too. this always help me to get the idea where to start.
Hope your get the solution for your trouble. Have a good day
In addition to Tim there's also the Fitness Activity Tracker demo here https://www.mendix.com/live/build-a-fitness-tracking-app/