Get yourself used to the interface. Move stuff, try stuff, tinker. Connect your phone via the companion, and see in real time what changes to the properties of components does.
Pick a component that you want to explore. Make a project, add that component, and in the blocks editor, add it’s blocks one by one, slowly. Understand what a block is doing before adding the next block. You’ll end up learning a lot about the component and what can be done with it.
Work as much as you find enjoyable. Getting stressed? Take a break. Try to get a little done everyday, and eventually you’ll have accomplished something great.
I recommend Firebase Realtime database for most stuff, and Cloudinary for media files (photos, for example). You can get started with these for free.
You can set read write rules to grant access to specific paths, or even the UID path. Easiest thing to do is to grant access only if user is authenticated.