Garages are a favorite hiding place for spiders because they are often dark and cluttered. Only a couple of North American spiders --- the brown recluse or black widow --- are dangerous to humans; most spiders are perfectly harmless and are actually beneficial because they reduce the insect population. But if you aren't comfortable with spiders living in your garage and would rather they find another place to inhabit, you can take steps to eliminate them and discourage them from coming back.
Eliminate clutter. Spiders like to hide in dark, secluded areas, so a garage full of boxes and other stored items gives them plenty of places to go. If you keep firewood in your garage, move the pile outdoors away from the house. Take items that are stored in cardboard boxes and transfer them to plastic totes with tight-fitting lids that spiders can't get into. Sweep or vacuum up dust and debris from the floor and get rid of any spider webs and egg sacs you find.
Reduce the number of insects that live in your garage. Spiders eat insects, so the fewer food options your garage offers, the less likely it is that spiders will want to live there. Some options for eliminating insects include the use of flypaper and keeping insect-attracting outdoor lights turned off when they aren't necessary.
Keep the doors and windows in the garage closed as much as possible. Place tight-fitting screens in the windows and over any air vents that may let spiders or insects in.
Seal off any holes, cracks or gaps in the walls, ceiling and foundation of the garage. In the majority of cases, these can simply be filled in with caulk or spackling paste.
Spray a spider repellent around the garage. Use a commercial chemical spray that is specifically formulated to repel insects and spiders, or try a homemade repellent solution consisting of natural ingredients that spiders dislike such as tea tree oil, peppermint or lavender, all of which are available from health food stores. Combine five drops of your chosen essential oil with one quart of water in a clean spray bottle. Spritz the solution over windowsills, along the walls of your garage and anyplace else that you've noticed spiders living.