How do you tell if the drywall has been painted? If you can see the color difference of the drywall mud where the nails and joint taping is, then it hasn't been painted yet. If it's a uniform color, it at least has a coat of paint on it. IMO, I would use a latex paint, they clean up with soap and water; the clean-up hassles of oil-base paints isn't fun for DIY'ers. You can use an interior or exterior paint, exterior will last longer, but do use a flat paint (gloss or semi-gloss will show every defect). I would probably paint with the garage door open, a lot of moisture is released from latex paints, and there is an odor buildup to some extent. Lower cost paints may require 2 coats, if the drywall hasn't been painted yet, you may require 2 coats with a premium paint which could be done with a primer and finish coat or 2 coats of a finish paint. Coverage is typically 300-400 square feet of wall or ceiling per gallon, but read the label to be sure! If you have oily or greasy marks on the wall, DO get an oil-base stain killer to spot coat these areas first. Auto parts stores have some spot removers for garage floors, you might check Autozone or someplace else.

Great! I appreciate the answers SOOOO much! I was just giving it one last check before leaving for lowes and you answered several of my questions.
The walls are just drywall off white w/the tape (looks like just white paint) in stripes up and down and some across.
I wouldn't have thought to get a flat paint though because isn't it hard to clean? When our house was built the whole interior was done in flat paint and every little grease mark shows up....everytime the kids touch the wall there is a spot! I guess I could go with a darker color to hide dirt better.
Alright, well I have alot of work ahead.
I started cleaning things out today and the floor has several grease spills and stains. My husband really is a bit of a slob!
He better keep it clean after this! Thanks again, jennifer