Carefully use a cotton swab dampened with Vitamin E Oil or baby oil to clean only the outer area you can see of your cat's ears. There may be a small amount of ear wax. Never poke anything down your cat's ears. If there is any problem, check with your vet. You can buy gentle herbal-based ear wash solutions or, in a pinch you can use hydrogen peroxide inside the ear canal.
Be extremely careful in cleaning the ear's outer surfaces as the capillaries in the ear are very fragile. If you are over zealous and manage to rupture one, a bruise will appear.