Here is a list of most used TCP ports which could be helpful is various cases:
The very first first step on securing a server is installing a firewall (atleast IP tables based) to close all unused or unwanted ports. Once the firewall is installed it is often considered 50% of work done. You can install CSF firewall or APF firewall. Often BFD (brute force detection) utilities comes with firewall.
ConfigServer Firewall, or CSF, is a common Linux security suite. Log in to your VPS as the root user and run the following commands to install CSF: