The three main nutrients you need to worry about are N-P-K(Nitogen-Phosphorus-Potassium). These are known as the macronutrients. During vegetative growth, a fertilizer high in nitrogen and potassium is your best bet. When you buy fertilizer you will see on the package a 3 number sequence such as 20-10-20 or 13-2-13. Those are always in the order of NPK. As you get to the flowering stage you will be best served by increasing the amount of phosphorus needed. Just make sure whatever you use, that you dilute it as recommended and even more on very small plants as the high concentration of nutrients can burn your plants.
If you get a fertilizer, it would be a good idea to choose one that contains the micronutrients as well. These are equally important to the macronutrients, but require less. These include, Iron, Manganese, Calcium, Sulfur etc.