Commercial fertilizers are easy to use. They are available in powder (dry), liquid, and just-stick-it-into-the-soil form - and should be chosen according to the preference of the user. Many offer application instruction for particular plants.
So, what are those numbers and what is NPK? The standard "complete" fertilizer is composed of nitrogen (N), phosphate (P), and potash (K), combined in various percentages (6-12-6 means 6% N, 12% P, and 6% K) for different plant needs.
Most office plants will be happy with a balanced fertilizer with equal parts NPK, such as 10-10-10. In fact, many plants will do just fine without any fertilizer at all. So use a light hand when fertilizing your plants. Use less than what is recommended on the fertilizer package - too much will do your plants more harm than good.