A lot of that depends on how you server is hosted. There are web sites to lookup IP addresses from hostnames, or you could just type "ping HOSTNAME" on the command line and it should show you the IP address.

As for finding out who did it? What are you going to do if you find out? I suggest you just use a better password next time. I would also mention it to your server provider just in case it was a more sophisticated hack, rather than just them figuring out your password.