The best way is to explore the various colors and styles. Light, medium, dark can all work, just what you are most flattered by. Likewise with the length. Unfortunately this is severely limited when your only shopping location is the internet. If at all possible try to find a wig shop where you can spend a few hours trying on different hair with the input of an experienced fitter. Otherwise, its a crap shoot and can leave you with a lot of hair you never wear.
Even if you can get the color and length right, there are different sizes and shapes of cap that can turn a lovely wig into a poorly fitting 'critter' on your head. Help is the best recommendation I can suggest.
Hugs, Robin