How to make MongoDB more secure?

I’ve been making projects with MongoDB for a long time but I never need to think about security part. Therefore, I’d like to know how I can make MongoDB more secure and unreachable.

My purpose of using MongoDB is website and game/project(user profile, project data, website data, etc.)
I use MongoDB in multiple dedicated servers for website and some game/project servers. For now, I do not use “sharding” feature. We can say MongoDB only works in same data center(same network).

What kind of things can I do for secure MongoDB in this situation?