Anti-theft system

I had to replace the alternator in my 2002
Cadillac deville. It won't start now. It says
service theft system. I have tried multiple
things to do this sometimes it starts but
when it does the dashboard and everything
goes crazy when I try to drive it and it feels
like it's going to stall out. when I try to start it
again it won't start. Still says service theft
system. How do I do this?