If you tank came with an Incadescent Light Fixture, then follow Joannde's advice.
However if you tank came with a Flourescent Light Fixture then you just want to get a bulb that has a Kelvin rating between 5000K and 10000K. You don't need to buy a specialty aquarium or plant bulb, a generic bulb from the hardware store in that Kelvin range will work just fine. Unfortunately 15 watts isn't a lot of light over a 10 gallon tank, so you'll need to stick to mostly low light plants.