Welcome to the ODK Forum. Please introduce yourself to the community when you have a chance - it helps us all understand your interest and use of ODK - which also makes it easier to find and provide help...
A good place to start is the tutorial in the Docs - in ODK longitudinal data is handled by 'entities' (the things you want to study over time):
There is plenty of background and also working examples of different forms / scenarios.
In fact, probably the first answer to your next question is, check the docs!