Complementary colours can be really tricky sometimes. Personally, I think every girl should be aware of the colour wheel but you don't have to wear, for example, yellow and violet together. These are just guidelines and you can pick what your eye is attracted to based on them. So basically the colour wheel is good to keep in the back of your head.
Complementary colours are basically colours that look good together (aka. complement each other). To get to know which colours complement each other, it's helpful to use a colour wheel. As illustrated on the amazing colour wheel that I found below. The colour across from the colour you pick is the colour that most complements it. I'm not kidding when I say that this definitely helps with the process of creating your outfit.
It's a really great idea to print a colour wheel and maybe post it up on the inside of your door, wardrobe or closet just to have it as a reference. It's little things like this that make fashion easier.
-Virgo