I guess if "someone succeeded" that they cracked your password. They may not have attained root access, but it's still violating when this happens. I guess you can formulate more secure passwords which you can check for strength online at websites devoted to doing that. Alternatively dispense with passwords on ssh and go for passphrases and keys, or just keys. They are extremely secure, and will still allow you to ssh into your server. You could change the port I guess in your ssh config file, but that might be inconvenient depending on the usage of your server. There's so many now automated programs used to do nothing but search for vulnerabilities, that it's near impossible to to "stop these guys". I'm not up with the latest fiddling with firewalls but I guess they have something to offer.
