There are lots of good guides online on how to protect yourself from this (disable password-based auth, or use 2-factor auth, Fail2ban etc.), but there's one piece of advice that crops up a lot that is probably a bad idea, and that's changing the SSH port. This can give (at best) a false sense of security and (at worst) actually reduce security if you choose a port number above 1024, because then malware could pose as SSH without requiring root access and then steal your password once you've entered it.
Just thought it was worth mentioning because I recently went through all this myself to try and secure a Raspberry Pi and it was news to me...
Just thought it was worth mentioning because I recently went through all this myself to try and secure a Raspberry Pi and it was news to me...