Maybe you have been thinking about hydroponics for a while but are bewildered by the shear amount of different types of lighting available to hydroponic enthusiasts.
Don't Panic Lighting is actually simpler than it might appear once you understand a few simple facts.

• The color "temperature" of a bulb refers to the color spectrum that it emits.
  Temperatures for hydroponic purposes normally range from 2700 (High Pressure Sodium) to 4200K (Metal Halide).
  At the lower end of the scale, HPS emits a warm color (red) which helps plants to flower/fruit.
  Metal Halide on the other hand emits a very cool color (blue) which is ideal for growing leafy vegetables and most house plants.
  
• There are bulbs available that are balanced to provide the best of both the red and blue spectrums.
  These are normally called daylight bulbs.
  
• Which bulb you choose is directly related to what you would like to grow.
  

How Many Watts Do I Need? Wattage is important and determines how much light your plants will receive (and how fast they will grow).
As a general rule:

• A 1000 watt light will be OK for a 7 x 7 ft growing area.
  
• A 600 watt light will be OK for a 6 x 6 ft growing area.
  
• A 400 watt light will be OK for a 4 x 4 ft growing area.
  
• A 250 watt light will be OK for a 3 x 3 ft growing area.
  

The above assumes that you have no natural source of lighting in your growing area.
If you are planning on growing cacti or some other high light needing plant, you may require more wattage than this.
The lights you choose to buy will depend on the type of things you would like to grow.
For example, are you planning on growing just foliage type plants (most houseplants)? If so then you would be looking to purchase a metal halide fixture.
For most growing needs, I would recommend at least a 600 watt system.
Remember that you cannot run a 600 watt bulb in a 400 watt ballast, so it's always better to think bigger than you need rather than ending up buying another ballast later on.