When first considering using a grow light you have to think about the size of the area you want to light. The wattage of the grow light that you need will be determined by the square foot area of your growing area. Some simple rules of thumb are 25 to 50 watts per square foot of growing area. So, you can plan on a 3' x 3' area using 250 watts, 4' x 4' area using 400 watts, 5' x 5' area using 600 watts and a 6' x 6' area using 1000 watts. If you have a larger area you can add multiple lights. This can be adjusted to fit your particular situation. Be careful of trying to place too large of a light in your growing area. The excess heat and intensity of the light could damage your plants. To avoid the excess heat from a larger light you may have to choose a reflector that can be air cooled. Air from outside of your growing area would be pulled in to flow over the bulb then vented out of the growing area via an inline fan and ducting. The bulb and the ballast have to be of the same wattage, ie. 250 watt ballast - 250 watt bulb, 600 watt ballast - 600 watt bulb.
If you have mylar or black/white plastic film on your walls or the wall painted white will help increase the light available to your plants by reflecting it back into them. Using a grow tent or indoor greenhouse will work the same.