What is the best paint to use for my plywood garage walls ?

Prime first, then paint.

You need to prime them first if they are new or have not been painted for a good while. There are many levels of paint now that help cover wood in only one painting. We buy ours at Menards for about $30 a bucket because it requires only one cover and stays for 3 or more years. It is excellent quality. Light grey with a dark grey trim would be really pretty!

I did mine in a light gray which helps hide better than white. For me, the cheaper the better! Walmart's brand is about $15 a gallon and I've had good luck with it. Go with a satin finish, it's easy to clean and hides small imperfections. :)

Wash them with TSP, prime with Kilz or Zinsser, then paint! Paint store will help you choose the correct formula and finish.

I would use a standard exterior paint from your local home center. Get a water-based paint for easy clean-up. Sine you are going with a light color, you may want to use one coat of primer first so you don't need as much paint.

Do I understand that you don't need the Garage walls to be Waterproof?

Definitely prime first. A design or grid is easy to do with painter's tape to give it some dimension. Tell the person in the paint department what you are using it for and they should be able to help. Whoops paint (paint that was mixed and then not purchased) is available if you ask for it, and can provide your base coat or finish coat if you like it.

add some grout to your paint, it will thicken the paint & should camoflauge the plywood.

Thanks everybody!!!