The basic thing is to get Burnt Sienna and Titanium White and mix them together. The Sienna goes from brown to peachy!
For more slight variations in color, you can mix in a tiny bit of Red Oxide and Yellow Ochre.
Enjoy! And if you have any other tips for skin-tone painting, please leave them in the comments!
Update: This mix of colors will give you a nice base coat. However, if you're going for a more complex and accurate set of skin tones, you'll need other colors too!
As Noadi comments: "In indoor light someone with fair skin will be very peachy and saturated with shadows skewed purple (not actually purple but in that direction) while in overcast daylight will be lighter and a cooler less saturated shade with shadows skewing towards blue-gray. Skin has a wide range of variations one just one person's face, you might need greens, yellows, blues, or purples along with the browns and reds you usually associate with skin."