The first thing you need to do before picking out colors is to decide if you have a Warm, Neutral or Cool skin tone. One way to do this is to take a good look at yourself in a mirror in natural sunlight. Decide if your skin color has more Blue or Yellow to it, or if they are pretty well even.
More Blue = Cool
More Yellow = Warm
Equal = Neutral
Another way to find out what skin tone you have is to compare your skin to the colors Silver and Gold. Generally if your skin looks good with Gold, you have a warm skin tone, Silver means cool, and if it looks good with both, then you have Neutral.
Sometimes it can be hard to tell on yourself, so you may want to ask a friend to help you decide.
Once you have decided your skin tone, it is time to choose a Concealer, Foundation and Powder. Depending on where you buy your cosmetics from, you may have a hard time figuring out which one looks best