I am a beginner and I am planning to develop my own app and I don't know how to work use 1 database on both mobile and web

I am thinking of developing an app and I don't know which service I should use for my data to run to. I tried on using Amazon Web Services but they don't offer any support for free users. And I am just a teenager who wants to develop my skills, I don't have any source of income at all. I am hoping some recommendations or help from here. Some ideas on what tools I should use.

My app is both web and I'll be making mobile app version of it and I am having a hard time about how would I make those two work from the same something like server. It's like, facebook website and the facebook app, it has only one source of data.

I hope anyone can help me...