When I first got my set, I bought a basic set of loose colors to form the color-wheel, and a few extras. So I'd get the ROYGBV set (Red, Orange, Yellow, Green, Blue, Violet), plus a brown, black and grey. After you get your basic colors, it's good to expand your collection to get different shades depending on what you do. Since I did a lot of skin work, I also got the skin-tone markers too.
Hope this helps!