API or MYSQL or FIREBASE for my app

Please can you give me your suggestion on witch would be the best for me to learn so I can build my app.

User will login to the app.
Pick out a subject, sub-subject, sub-sub-subject.
Then get data with two buttons. One to end and the other to get more data.
Will go on until the user hits the end button.

Have the user be able to make the subject, sub-subject, sub-sub-subject.
Plus be able to put in the data. Data could be a photo or text and photo or just text.

Please can you give me a list of thing to study (videos).
Thank you

Checkout https://approadmap.io

It’ll help you figure out what all you need to learn to make your app!!

I’d suggest either Firebase or MySQL. Depends on which you’re not familiar with but either could work.

An API isn’t a storage solution.
More details about an API here