This is a very common question from both novice and experienced fish keepers alike and we are going to try to answer this as simple as possible. There is one main question you need to ask yourself when selecting a light:
What will I keep in my tank?
This is very important as different types of fish & plants require different types of light. Without getting technical and going into spectrum or colours, work out what type of fish you want to keep. Have a look at the table below and this should make life easier: