I'm 20 and I stabbed a homeless man
I hope you are joking! Why would you commit a senseless act like that?
Because my boyfriend gave me thumbs up for it.
Regardless of whether anyone gave you the okay to do it, you shouldn't stab another person unless in self defense.  Random acts of violence are not okay and will most likely land you behind bars.
What can you do about it?
It can be reported to the police for one. It's wrong to stab someone no matter who it is or who told you to do it. You need to think about your life first.