With so many options out there, it is often easiest to make a start with a few simple "rules of thumb' such as "What face shape do I have"? Once you know - you know and then it is easier to reason as to why a certain shape works or not.
Simply take a photo of your face, remove the colour and maybe even reduce the transparency, so you can "see" the overall shape. If it still isn't an obvious Oval, Round, Square, Heart or Oblong then you can do the following analysis:
Use a grid or a ruler and look for the WIDEST part of the face, is it the jaw line, the forehead or the cheeks? Then compare to the other parts of the face, is one part dominant, or are they similar?
Next take a look at the LENGTH of the face in proportion to the width, is it about right, slightly longer, or shorter?
Lastly take a look at the JAWLINE, is it strong and square, or rounded, or pointy?
If you can't figure it out, ask someone else for their opinion too and use these guidelines to help determine.
Once you have figured out what shape you have then you can begin to make some decisions. Basically whatever shape face you have - purchase sunglasses that are the opposite, have a round face, don't choose John Lennon type round glasses, use a cat eye. Have a heart shape face, go for aviators to balance out the forehead and to add volume to the chin. Same with a square face, choose a cat eye or round shape - don't "repeat" the strength of your jawline.