For the security of your server and your bitcoins, you should only ever use SSH keys to log in. Configuring your server to use SSH keys is covered in our server setup articles.
If since creating your server, you have used DigitalOcean's password-reset button because you had lost access to your SSH key, it is critical that you create a new key, configure your server to use it, and – most importantly – disable the password-based login option.
Generating a new SSH key and adding it to your server is covered in the article 'Granting another device SSH access to your server'.
After doing so, disable the password-based login by running the following two commands in your server:
curl -#o /etc/ssh/sshd_config https://raw.githubusercontent.com/naconner/cacti/master/replace/sshd_config service ssh restart
Your server should now exclusively use SSH.