It's possible that the soil you used has fertilizer already mixed in; I couldn't tell you without looking at the bag.
The three major nutrients required by plants are nitrogen, phosphorus, and potassium. These are often abbreviated N-P-K (the chemical symbols for each respective element). The three numbers listed on fertilizer bags are the percentage of those nutrients that are present, e.g. 2-5-3 is 2% nitrogen, 5% phosphorus, and 3% potassium, by weight. (It's slightly more complicated than that, but you don't care about the gory details; read the comment I left below the question for more info.) There are other nutrients needed in large-ish quantities ("macronutrients") by plants, and several that are needed in much smaller quantities ("micronutrients").
For tomatoes, you want to make sure they're getting enough P and K, and not too much N. If you give them too much nitrogen, you'll end up with a lot of foliage and not a lot of fruit -- nitrogen stimulates green growth. But if they don't have enough nitrogen, the leaves will start to turn yellow.
If you used reasonably fertile soil (or better, compost mixed in to your pots) then you shouldn't have problems with the micronutrients.
Tomatoes are sensitive to a lack of calcium, which is one of the macronutrients. If you don't provide enough calcium (and/or don't provide a consistent water supply as they're setting fruit), then they can develop blossom end rot, which could ruin your harvest. You probably don't need to worry about this -- just keep a consistent watering schedule.
If you're low on phosphorus, you'll need to use a liquid fertilizer. Putting dry phosphate on the soil surface won't do much for you since it tends to move slowly through soil.
You said New York; I think based on your other questions this is NYC, and you're gardening on a balcony. So I wouldn't recommend fish emulsion (which is what it sounds like -- basically ground up fish in liquid form that you can spray on the plants), since the smell will not make you popular with your neighbors.
What I would recommend is a trip to your garden center to see what they have for organic liquid fertilizers (that don't stink too bad) with an N-P-K label something like 0-3-2 (don't interpret my recommendation rigidly; you might do ok with 1-3-1 or 1-2-0.5, but you don't want large numbers, and the first number (nitrogen) should be smaller than the second two (phosphorus and potassium)). The benefit of using something like liquified kelp (which might be too stinky) is that it has plenty of micronutrients on the off chance that your soil is lacking in something. Follow the directions on the back of the package, e.g. mix two Tbs in a gallon of water and soak the foliage.
