Your first course of action will be to identify your skin's undertone. This will either be pink or yellow, yellow being the warmer tone and pink the cooler tone.
A good way to figure this out is to look at the colour of your veins. Looking at the wrist is the best way to do this. If they are more green than blue, you have a yellow undertone. More blue than green, it's pink. If your veins look both blue and green then you most likely have a neutral skin tone, which means most all shades will look great on you!
pink undertone
yellow undertone 
So, for yellow tones, try a warmer lip shade, and for pink tones go for cooler shades.
Step two:
Take the occasion into consideration:
Professional - For work situations, you should try to pick a colour just a few shades darker than your natural lip colour. That being said, if you like to go heavier with your eye make-up, stick with a nude lip.
Bridal - For your wedding make-up, a classic red lip colour is often the best way to go. Again, be sure to take your skin tone into consideration when choosing the best red.
Active - And no, I don't mean the gym (where you should really try to wear as little make-up as possible!). For active pursuits, try a neutral lip colour. Nude is the key here.
Party - This all depends on your personal style. For glam, a glossy or glittery colour will work best. If you have a bold style, go for a red, orange or pink lip. For a grungey/alternative look, choose dark plum shades. Try picking a shade from the NYX Glam Lipstick Aqua Luxe collection.
Date night - Rose colours are beautiful in soft lighting, reds and soft pink also work here. Avoid an over glossy look.
Try a shade from the NARS Final Cut Lip Pencil collection.
Et Voila! You should now have the knowledge you need to choose your perfect lip colour!
