You question is too wide as previously noticed. The best way in general is not to look for 10 tips for improving security, but to dig into linux administration. General rules are:
- minimum priviledges to do the job (take advantage of basic permissions
- is you connect to external APIs you are exposed (so what?), if you provide API than you are exposed (so what?)
- do not try to make general protection - know your important data, keep important data secured and encrypted. rest of them - do the backup
- do not be lazy and install the solutions for security (do not take antivirus seroiusly)
- play around with firewall (iptables, firewalld ...) try to access from outside and try to reach another server knpwing exactly what you're doing (much harder than you'll expect)
- get any book for linux administration (Red Hat books are preety good and comprehensive
- do not be affraid - just keep learning and trying. cPanel is great tool (sometimes annoying with default settings). If you are not running unmannaged dedicated server - ask support - they always answer precise questions. (unmannaged do not answer even in paid support)