I believe Johnny's just always been guitar (mosrite, some sort of replaceplament pickup I believe - basically, hot humbucker pickup) into Marshall stack. Set to "Loud".
So basically, overdrive yr amp head. When I saw the Ramones, Johnny was playing through two Marshall stacks. None of us probably need that for practice, or even with our band though, so get a smaller tube amp head (or build one!) and just push it! Use a booster in front to help smack it around. We all know the drill by now. As far as perfecting that tone, I feel like he uses the neck pickup for some reason, but I could be wrong. I know only one pickup in his guitar supposedly worked.