Water is pouring out of the top of my hot water system, can I pull the relief valve to stop it?

I pulled it a little, but not sure how hard I am supposed to pull the lever?

Turn off valve on the cold water input line.

When you say "pull" you mean to unscrew it and take it off, correct ?

Yes you can do that - but you have to install new one. You cannot just plug the hole. Leaking relieve valve indicates one of two things: high water pressure or high temperature of hot water. You may have to adjust or replace pressure regulator and/or install expansion tank. Back to you. Milo