How to sync app to other devices, data backup etc.?

I've been researching about this for quite a while and got shocked by immensity of the information about it. First, I am developing an app and I am on my way on persisting the data from my app (which is a db) to the web. I can't figure out how it all works, do I just upload the db file onto a server? I also plan to sync across all devices. So when I edit an info on one side, the other device can see it (when online). The app should also work even if offline and push changes on the local data to server when it become online. I don't like my app having a stale info.

Where do I start from here? I've been checking Realm, Parse, GWS, AWS, can't get to seem to figure out what their services offered best for me. I am so dumbfounded on this aspect of my project. Sure I could write apps, but I am just stuck being offline.

Any enlightenment is appreciated.

Thanks!