I want to shoot up my school real soon
Are you playing with me? Why do you need to shoot up the school?
I want to be an outlaw.
A school shooting is really serious. If you're not joking about this, you should definitely talk to a professional or a trusted adult if you're thinking about hurting people. If you're joking, that's a really obnoxious joke and could get you into some trouble.
I was joking about the school shooting but if it happens I may be the next shooter.
Joke or not, you are still considering it so I feel it's best that you tell your parents how you are feeling so they can take appropriate action on your behavior. No one deserves to die, especially young schoolchildren.