You either have a bad ignition switch, a short somewhere, or you need a new starter. If I had to guess I'd say a new starter. You need to bypass the "start" part of the ignition switch by directly jumpering the starter, (if you don't know how to do that, I'd say take it somewhere), the starter will engage, then if you remove the jumper, the starter should disengage and stop. If it doesn't, you need a starter (or a rebuild), if it does, there is a problem in your ignition switch. Good Luck.