Soil is VERY important. Bad or cheap soil (ie. Miracle Grow) can cause a number of problems when growing Cannabis such as nutrient based burning and insect infestation.
I would HIGHLY recommend Fox Farms Ocean Forest Soil, as seen here:
http://www.foxfarmfertilizer.com/prod_big_oceanforpot.html
As far as watering goes; Over-watering is the most common mistake that beginner growers make.
Here are two simple, but effective, methods for knowing when your plants are ready to be watered;
One: Stick your finger in the soil about an inch to an inch and a half down. If you feel any moisture at all then DO NOT water.
Two: This is the most reliable method as long as you are on point. Pick up the plant before you water and get a feeling for about how much it weighs. Every few days pick up the plant and when it is the same weight as when you picked it up before you watered then it is ready to be watered.
Letting your plants dry out completely in between each watering is very beneficial to the plant because it causes the roots to grow in search of water.
Remember, Bigger Roots = Bigger Plants & Buds
I hope this helps.
