How do I replace a dimmer switch with a single switch?
To make the switch work, you'll simply attach the two wires you removed from the dimmer to the new switch. If there wasn't a grounding conductor attached to the dimmer, you have a couple options for grounding the new switch.
Add a grounding conductor
You can use a length of bare copper or green insulated wire, and connect it in an approved manner to the other grounding conductors in the box. Then you'll attach the other end of your grounding pigtail to the switch.  You could use a twist-on connector, crimp connector, terminal block, push-in connector, or any other UL listed product designed for this.
Adding a grounding conductor is the preferred method.
Self grounding device
If it's a metal box, you could use a self grounding device.  The switch will then be grounded to the box through the attachment screw, assuming of course that the box itself is grounded (which it should be if it's metal, but don't make assumptions when working with electricity). Self grounding devices will be labeled as such, and will have a little copper doohickey around the attachment screw hole.

NOTE:
If you're attaching a grounding pigtail to the other grounding conductors in the box, make sure all the existing grounding conductors stay connected. DO NOT permanently remove grounding conductors from the bundle, they must all be connected together.