First time growers frequently:
Overwater their medium based plants. When you pot your plant, judge its weight dry by hefting. Then, water the plant thoroughly, until water runs through the drain holes. Heft it again. When your pot feels as nearly as light as it did dry, its time to water again.
Overfeed Their Plants. As Mr Soul says, beginners rarely under do anything. When all else fails, follow the directions on the fertilizer bottle.
Use Poor Quality Cannabis Seeds of unknown origin and waste money and effort, where 90% of the final crop amount and potency comes from high quality marijuana seeds genetics. Order your seeds online using my guide on how to buy marijuana seeds.
Over Analyze Their Grow. A first grow is like a first born child: you pay attention to every little thing that happens. Further, you worry at the first sign something may be wrong. Pay attention to what happens in your grow, but do not try and find a remedy for every yellow leaf. Frequently, the remedy-like flushing your medium-causes more problems than it solves. Be responsive when things go wrong, but be conservative in your remedies.
Overspend on Grow Supplies. New growers frequently commit themselves to unrealistic and expensive first grows. It is much more efficient to learn how to grow marijuana and then invest in high end equipment rather than the other way around. Most experienced growers don't have room for all of the grow paraphernalia they aren't using. As you will see throughout this FAQ: K-I-S-S.
Overpost. Try the search engine first. Chances are good that any question you can think of has been asked and answered before.
Germinate the Cannabis Seeds in Pots, while the proper procedure for germinating marijuana seeds is the paper towel method between 2 plates or in a plastic bag.
Talk About Their Grow. Don't tell anybody that does not have to know. How can you expect anybody else to keep a secret that you cannot.
Courtesy: 1st Marijuana Grower's Page