Unless your soil is perfectly balance and rich in nutrients you need to fertilize periodically. How much and when depends on your soil and climate. All fertilizers have a row of numbers listed on, them some with a letter in front of each. These letters are symbols for the nutrient content of that particular fertilizer.
N is always first on the list. This is for nitrogen. Nitrogen helps keep plants green. It helps the plants produce and maintain chlorophyll, which they need in abundance to take up nutrients. If you are wanting lush green growth look for a high number in this space.
P, the second letter, stands for phosphorus. Phosphorus is for strong roots and good blooms. Use a fertilizer with a high phosphorus number for plants that you grow for their blooms, or for more fruits.
Last is K. K stands for potassium or potash. This nutrient is needed for general well-being of the plant.
Most organic fertilizers will have lower numbers than their chemical counterparts, but they work very well. NPK is only part of what a plant needs. There are tons of micro-nutrients that they need as well, many of which can only be found in organic fertilizers.
The most important thing to remember is that more is not better with fertilizers. You can easily burn or kill your plants with too much “tender loving care.” Just follow the directions. For seedlings use 1/2 to 1/4 strength fertilizer, as they are very tender at this stage.