The first step to choose bicycle lights is to define the need for it. Be specific here as the rest of the process depends entirely on this step. Do you need the light to cover a large area? Do you need the light to cover more distance ahead of you or to the sides? Do you need the light to be powerful? Do you need the light to be able to alternate between low, medium and high settings? Do you need the light to be a strobe light?
A number of questions will be raised but answer the ones most important to you and note down the top three reasons, which you feel are the most important and keep those in mind when you actually go out there and begin your quest for the light.
The next step is to do a bit of a survey. This can be done either online or in person by visiting various shops. See what is on offer. Chances are that you will find what you are looking for but keep looking anyway, you may end up finding something better.
Once you have a list of potential lights based on your requirements, check to see if they are compatible with your cycle. You do not want to purchase a light that cannot fit on your bicycle, as that would be a waste.
The final step is to see whether or not the light can be charged or is compatible with a dynamo. The best kind of light is a combination between the two, as it alternates from one source of power to the other.