You're currently on:
The Real Poop On Fertilizers
An overview of plant feeding
Generally it's a good idea to mix a time release or dry fertilizer into the soil before planting. Then use a liquid fertilizer throughout the growing season to replenish water-soluble nutrients that are washed out of the soil by frequent watering.
There are many organic and conventional fertilizers on the market today to choose from. Look for blends suited for the type of plant you are growing - leafy, flowering, vegetables, etc.
Keep in mind that more is not better when it comes to applying fertilizer. Too much fertilizer in contact with your plant's roots will draw the water out of the root cells and burn the plant. Always follow the directions on packaged fertilizers, and read up on your plant's specific fertilizer needs.