If you want to have your own space where you can escape to work on projects and watch the game with the guys without interruptions, consider transforming your garage into a man cave. The first project you'll likely have to tackle is to make your garage livable. You want to make sure it's not so cold and miserable that you never want to actually go out into your garage. Heaters specially built for garages can help.
Start your garage heater search the same way you would any other appliance: choosing between gas and electric. Stick to whatever system your home heating is hooked up to. Then, cross out any heaters that aren't marked for residential use. Though upgrading to a utility-grade heater may sound epic, a building inspector won't let it pass, and you'll be paying for more machine than you need.
You also need to be aware of space issues. Garage heaters are typically mounted on the ceiling, so you'll need to compare the dimensions of the unit with the headroom you need to effectively use your space. And don't forget to count the drop space needed between the unit and the ceiling. Finally, get a good measurement of your space and take stock of your insulation. Much of the decision-making can be taken care of based on the pre-existing factors in your garage.
Choosing a garage heater is fairly simple; installation, however, can be very tricky. In this case, it's much better to leave it to the professionals. Installing a garage heater deals in many dangerous points: tying into gas or electric lines, dealing with a heat source and fire dangers, and working with a building inspector. Unless you are experienced in all of these areas, write the check and forget about it.