Your basic nutrients are NPK, Nitrogen, Phosphorus, Potash (potassium).
They each have their own area which they specifically help, so once you know what they are, think about the qualities of the herb/fruit/veg/plant you're trying to grow, and that will give you a strong indication of the balance required.
One thing I would say though, is that, in soil, you don't have the ability to screen out any nutrients which are there, and as such a balanced fertiliser like growmore or blood, fish & bone, is likely to give satisfactory results.