The prep for your garage interior is very basic and easy. I have painted garages just like yours, here is what I have done to get it ready.
1) All walls and the ceiling will need a good stain blocking primer. Probably the 20 year old raw drywall has discolored and might have stains. I have used Kilz Original Oil Base with good results. You might have to apply two coats to persistent stains.
2) Fix any holes or loose drywall tape. Typically the walls and ceiling is only taped and first coated with drywall mud. If this is OK with you then you are ready to paint. If you want a better look then you will have to spend more time coating the seams, nail/screw heads and joints.
3) You can use any good paint. Interior or exterior, it doesn't matter that much. I recommend a satin sheen for easier cleaning.