One of the smartest things you can buy for your makeup bag is a good moisturizer. And, actually, you may want a few.
First, everyone should have a light, moisturizing day cream -- look for one that is specially designed for the face, and one that contains SPF to help hydrate your skin and minimize sun damage. Apply it after your morning shower and before you apply any makeup. (If you forgo concealer, look for a moisturizer that is tinted to help even out your skin tone.) A good facial moisturizer can also double as a good eye cream, but if you're worried about minimizing fine lines and wrinkles or reducing dark circles, pick up an eye cream designed to treat those problem areas.
And finally, night cream. Night cream will help treat dry skin and fine lines, help even out any discoloration and brown spots, and help restore an even texture and brightness to your skin. Be sure to wash your face and remove your makeup before applying.