When it comes to buying a front light for your bike there are a few things to consider. Firstly do you intend to ride off-road or on road?
For road and commuting consider a set of lights that can be recharged via USB - a great advantage for charging at your desk. A front light with between 100 and 1000 lumens is ideal. If the roads or areas you are riding do not have any street light then the higher the number the better. A secure fastening is a good idea and maybe a quick release clip system for easy removal when not in use. Lumen wise the front should be a powerful light with a wide beam simply to show up potholes, rear wise you can get away with a large surfaced light and a lower lumen count, or a more powerful light. Also choose lights that have a few flashing functions, because remember that guy you saw flashing like a Christmas tree? Exactly you saw him.
When it comes to off-road lights a minimum of 1000 lumens mounted to your bar is ideal. For ultimate trail coverage and visibility, you are better off doubling up with a helmet mounted light of around 1000 lumens. The beam should be as wide as possible as the more trail you can see the better.