Your ignition switch needs to be replaced. Some people have had luck repairing theirs, but I actually made mine worse when I tried. You don't need the entire key and lock mechanism. If you take your the plastic covers off behind the steering wheel, you can get to the actual switch. It looks like a white cylinder with some wires coming off of it on one side, and the other side is attached to the key mechanism.
Here is a pic of the complete part you need:
Here is a pic of the complete part you need:
