First, find a price range to know where to begin the search.
Second, establish the skin type you have (oily, dry, acne, combination?)
Third, Match your skintone
Fourth, Go to the store and consult with a specialist on what make-up brand would be suit you (wrinkles, medium coverage, sensitive skin?)
Always apply a small sample of foundation to both the top of your hand and to the underside of your wrist, this will allow for the closest match to your natural skin color. Take into account your skin type, oily versus dry, and choose the foundation type that works for you.
Leave it to the professionals and go to MAC cosmetics and they'll pick the perfect shade. If you go home and decide you don't like it; you can return with your money back.
You might go to a department store make up counter and have them help you find the right color to match your skin. Or, to a store like Sephora where they will give advice. Or, a trusted friend who wears makeup and will help you out.