The way easiest database I’ve found to get up and running with is the Sandbox version of MLab (which is hosted by Mongo). This is a cloud hosted database that you can read and write data to. It’s easy to implement and it’s FREE!
For impleneting a database into your existing Digital Ocean hosted project here are a few guides for installing a database on DO. This is a bit more complicated than just using the free cloud version, but not too bad and more flexible overall.