The first thing you should do is build a server in your home and set up a network. ssh into that server and play around where you don't care if you do any damage or not. Learn some linux commands. And when you are comfortable doing that then try logging into your real server.
Logging in as root is very dangerous if you do not know what you are doing. You asking the question about how to login as root through ssh proves that you do not what you are doing and are likely to really mess something up and break your server.