There are lots of ready made nutrients on the market some aimed at cannabis some not. In the end there's not much difference between any of them. As long as the nutrients are complete and given in the correct dosage results should be good.

 Cannabis is a nitrogen lover and whilst in its growing stage it will need a feed with extra nitrogen. When flowering starts it will require extra phosphorus and potassium instead. This is why a lot of cannabis nutrients come in two or even three part formulas.

 If a plant looks under fed check the p.h. of the medium and water before adding more nutrients. If your growing in soil make sure your not over watering and suffocating the plants roots. Soil should be allowed to dry a little between watering.

 To easily control the rate and amount of food a plant receives use bland nutrient free mediums, with or without standard potting compost mixed in. Then add a soluble complete feed to the plants water in the recommended amounts.

Some nutrient ratios: 

 Nutrients have an N.P.K. ratio on the packet. This shows the amount of Nitrogen(N), Phosphorus(P) and Potassium(K) the nutrient contains as a percentage. A complete fertiliser will also contain all the other nutrients and minerals a plant needs for good growth.

 Mixing organic nutrients takes time and knowledge. Getting a good safe balanced mix for plants grown in the ground can take months. Start planning now if you want to grow organic and remember that you may still need to feed with a nutrient solution as well.

 Whatever nutrient you use you should always stop feeding at least 10 days before harvest. Only give the plant p.h. balanced water from then on. Doing this will help "flush" the nutrients out the plant and produce better tasting and better burning buds.