There are a lot of aspects around security that relate to how to secure your OS, which by default is very locked down. What applications you install and how up to date their versions are, such as Apache, Sendmail, MySQL, etc, or pretty much anything that opens up an external port. And lastly, what code you write that is remotely accessible.

So it’s an extremely large subject.

However, to get started you can browse over 200 tutorials on Security over here:

DigitalOcean Security Tutorials