The Foundation of Your Look.

Consider if you want a light or medium-coverage. If you have good skin, but want to even out a few ruddy areas, consider a light coverage. For women with really uneven skin tone or blemishes, consider a medium-coverage foundation. Some women swear by mineral foundations, so you might want to try these out as well.
Show up to the makeup counter with a clean, fresh face. It's OK to wear eye makeup or lipstick, even moisturizer on your face, but you don't want to show up already wearing foundation.
Select a few colors that seem close to your natural skin tone. Go for the yellow-based foundations, which look most natural on all skin tones. However, if you're very fair, try a pink, cooler shade.
Apply a stroke of up to three colors along the JAWLINE (the inner wrist or hand aren't the best spots, they are NEVER the tone of your face). The one that disappears is the right shade.
Take a good, long look -- near a window (natural light is the best for deciding). Ask to borrow a hand mirror and stand near a doorway, or step outside, to see which foundation looks best in the natural light. The right shade is one that you can't see because it blends in so well. If you are purchasing from a drugstore, take a couple bottles to a doorway, hold them up to your neck and see which one matches the best or if you have a favorite shade you want to match, match the colors of the bottles, never the swatch on the display.
Once you've bought the foundation, take it home and try it out. If you don't like it you can return it. If you're buying from a drugstore, purchase from one with a liberal return policy (like CVS, Walgreens, Walmart and Rite Aid).

Ok, that was from a makeup artist stand point, but as an esthetician, today foundation is totally optional. It's healthier for your skin NOT to wear foundation daily. If you have blemishes or pimples, try strategically placed concealer instead. Or just wear the foundation on the parts that need it most, likely around your nose and eyes and on your chin. I try NOT to wear any at all or I lightly apply it like I would do bronzer...more natural looking. Just make sure to blend properly.

Thanks for reading and have a marvelous day!