There are a million things to do already when building a product and Firebase easily takes away half of that functionally by covering most of the things you'd need for a backend, plus it's free (mostly). Start here and figure out where to go later because if you've outgrown this you've already done amazing :)
Simple to use and backed by a bustling community that seems to produce amazing work. Some tiny difficulties occasionally but well worth spending time to learn it for the problems it solves.
Tip: Don't dive too far into state management and best architecture but focus on getting stuff out of the door!