Ask HN: Cheapest cloud DB service?

Hey gang, please help a front-end dev out. I need advice on the best DB SAAS/cloud service. I have an upcoming side project that will - if my assumptions are correct - be pretty popular, pretty quick. At least to some segments of the population. It'll be a free(mostly) service that requires a heckin lot of DB reads. The data being passed around won't be big or complex, but I'm anticipating really, really frequent reads and moderate writes. I guess a similar read/write equivalent would be a messenger app. My day job is mostly front-end JS development; React, Redux, those kinda things. I have enough back-end skills to get me into trouble, but I want to plan this out right. If I'd need to scale the service up quickly, I'd need it to be in a position where it would be as fast and cheap as possible. So what kinda DB would I need to look at? SQL or NOSQL? Which service provider would give the most bang for buck? Amazon AWS? Google? Azure? Does DigitalOcean have a solid db service? What caching solutions should I be looking at? Anything else I might not even be considering? Please keep in mind that it'll just be me as developer(so the learning curve should me moderate), and that I'm, shall we say, on a really tight budget.