My non-production Postgres DB got Hacked ... what are some of the good strategies to secure the Postgres DB on Ubuntu 16.04 ?

I take full responsibility for my lack of efforts for securing my server. I took no precautions to secure my Database. But still I wanted to make everyone aware that my Postgres DB on Ubuntu 16.04 was Hacked. and hacker asked 0.15 Bitcoins be deposited in his account in order to restore the DB. Fortunately my application is still not in production and nothing Essential was lost.

But it does raise few important concerns in my mind and i want to know what kind of strategies and precautions I must take in order to protect my Postgres DB on Ubuntu 16.04 ?

So that such a thing does not happen when my app goes into production.