You should ensure that (1) password authentication is disabled, (2) existing ssh keys are recycled (if only password authentication was used earlier, generate those [2]).

Subsequently, refer to "Essential Security for Linux Servers" [0] and "7 Security Measures to Protect Your Servers" [1].

[0] https://plusbryan.com/my-first-5-minutes-on-a-server-or-esse...

[1] https://www.digitalocean.com/community/tutorials/7-security-...

[2] https://www.digitalocean.com/community/tutorials/how-to-set-...