To protect yourself against this type of attack succeeding, namely to guess your password and compromise your server, you can choose an extremely long password utilizing many symbols %$!&^ and numbers 190921, both UPPERCASE and lowercase letters.
Alternatively, and the most effective way to protect yourself against these attacks is to change the port with which your SSH service runs on. However some caution should be taken when changing SSH ports of the server, as if done improperly this can cause you to lock yourself out with the firewall. The process is simple though;
locate sshd_config in /etc/ssh/sshd_config
Change port to something else like 777
Open up Firewall rule (very important)
sudo iptables -I INPUT 1 -p tcp --dport 777 -j ACCEPT
Save Firewall Rule
The entire process is described at:
Other alternatives ways to resolve this other than using secure passwords, or a less obvious port than SSH’s default port 22, is to install something like fail2ban. Which will ban any IP address preventing it from logging in if it gets the password wrong a certain number of times in a row. However, pleas be careful with this too as you can lock yourself out if you are not careful, as with all security software.