How do I paint card board ?

You can use oil base crayons, markers, paint brushes and paint, or spray paint. They will all work for you, Susan. What are you making?

With acrylics!

Just buy some foam brushes and use whatever color you want and paint away. Easy peasy.

You can use any craft or spray paint for cardboard.

Any paint will do. Just know if the cardboard is thin, and the cost of paint is wet and heavy , it might not dry flat.

A good way to make the paint not seep into the cardboard, paint a layer of mod podge, gesso, triple thick, or even school glue on the entire surface of where you're wanting to paint, let it dry then go over it with your paint

Repetive light coats of spray paint works for me. Let it dry completely between coats.