I grow in living soil, a retail product called EB Stone Recipe 420 Bhang. I don't feed my girls any nutrients at all when they're vegging. The soil has all of the nutrients a cannabis plant needs for at least 3 months. When they start to flower I throw in some of the organic Recipe 420 Ultra Bloom into the soil and they get a little molasses with their water.
Knowing the soil is good, I only need to worry about light and water. The Sun provides my light, and through the winter months I supplement with light in the shed to keep them in veg. So that leaves the water... And while what's left should be the easiest, it isn't always so. Plants need the RIGHT amount of water, and things like pH, chlorine and minerals can fuck your entire grow up. If you're using municipal water for your grow, you might want to have it tested so that you know what you're working with. For instance, you might be surprised to discover there are more than ample amounts of calcium and magnesium coming from the tap. And I know it's probably overkill, but for the water going into my plants, I add a small amount of vinegar to bring the pH down to 6.8, and I fill my waterung cans at night so that the chlorine and chloramines dissipate overnight.