A common mistake often made by first-time growers of marijuana is over-watering the growing medium that is holding new marijuana plants. The general rules are, when first potting your marijuana plant, give your best guess as to how much it weighs when dry. The best way to do this is by hefting. Then, soak your marijuana plant thoroughly until you can see water running out through the drain holes. Once the water stops running out, heft your plant again and notice the difference between the two. Once you heft the plant again and it feels similar to when it was dry, then you know it is time to water your plant again.
Another common mistake is putting too much plant food on the marijuana plant. If you are unsure of how much to feed, then just follow the directions that the bottle says; then you will not have anything to worry about.
Another common mistake that new growers tend to make is over-analyzing every little difference in their marijuana plant as it grows. This can be everything from thinking the plant is not faring well to thinking that you are making mistakes in the growing process. The best thing to do is watching how the plant is growing and still understanding that a yellow leaf does not automatically mean that your plant is dying. Watch for big signs that something is wrong and then try and see what you can do, but don't go so far as to flush the medium of a plant unless you know that is the only possible remedy for the problem. It often times causes more problems than it solves, especially when done to a plant that is just trying to adjust.
The best way to avoid this mistake is to take the time to educate yourself on the growing process before starting your first growth. This will allow anyone that is learning the process to buy only the necessary equipment and supplies and save the extra investment until the process is understood and easily replicated.
If you look around on the internet, you will likely find that all of the questions that you would really need answers to are already there and answered. Just make sure you only talk to people who need to know about what you are doing and no one else. The more people that know, the more likely it is that your growth will come to light.