If you want fresh air then you will need a fresh air intake, as well as a exhaust fan. Cut a whole, mount a all weather duct and metal exhaust fan from any local home depot, and wire it accordingly.
If you dont wanna rip into anything and want an easier way, you can do what my friend did. He opened up his garage door about 2", made a custom duct that fits under the garage door, and used cheap box fans from home depot to force air in, as well as blow air out. works AMAZING! When he is not using it he can just slip it out from under the door, and simply remove it. Just needs an extension cord.