My garage floor is concrete, it was previously painted years ago, most of the paint has gone through the tire heat. what type of prep must I do before applying Quikrete Garage Floor Epoxy Gray kit?

As long as it's Latex Acrylic based paint, anything that is loose or peeling needs to come off. You can then paint over-top of existing paint. However, follow this test. Put a piece of duct tape onto the paint and peel the tape off. If it doesn't pull the paint with it, it is ready to be painted. If it pulls the paint, then it must be chemically or mechanically removed first.