Posting pictures of your plants would help us help you.
Keep in mind that, especially amongst new growers, the single most-common reason for yellowing is overwatering. Simply backing off of your watering will likely cause your plants to green up on their own. A fairly good sign for new growers to follow is to wait to water until the leaves droop just a tad.
The age of your plants and your growing medium determines the right mixture of fertilizer. If you are using potting soil or bagged garden soil, you should be aware that most potting soil (not all, but most) and some bagged garden soil comes with fertilizer pre-mixed in it. Because of this, your plants need little additional fertilizer. Additionally, if you add too much fertilizer by using the directions on the label, you will burn your plants. However, if you are growing in the ground with regular soil, you may have more need for fertilizer. If your plants are still young, you will want to dilute the fertilizer more than recommended on the label.
Very young plants need very little fertilizer - their cotyledons ("seed leaves") provide the food they need until the cotyledons fall off. Even after that, they are somewhat like human babies - they can't eat as much as an adult can. This is part of where pictures would help - I have no idea how mature your plants are at this point, so can only make guesses regarding what your plants might need at this point.
