Lights on your bike serve two purposes: they illuminate your path in the dark and make you more visible to motorists and pedestrians. The type of lights you use will depend on where you ride.
There's usually enough ambient light to see, so your lights are primarily for making you visible to motorists, pedestrians, and other cyclists.
Comparatively low-powered lights work well for city riding:
Consider mounting two lights both front and rear. This protects against failure, and lets you operate them flashing out of phase with each other, so there's never a dim micro-second when a motorist glances in your direction. Wearing a helmet mounted light in addition to a handlebar light, lets you aim a beam directly at motorists who are approaching from the side.
Away from street lights, you must light your own path. The faster you ride and the trickier the trails, the more light you'll need. To give you time to react to obstacles, you'll need both a wide beam and a spotlight that throws light a long way in front of you.
If you are not going to use your light in the summer, or you park your bike over the winter, top-up rechargeable batteries before you store them. The batteries may be damaged by sitting around without a charge for an extended period of time. Pull them out a few times throughout the season and charge them for a few hours. It is also a good idea to remove non-rechargeable alkaline batteries from lights before storing them, as they can leak and cause damage.