Plant still young, first grow, very confused...

Before starting a grow it might be a good idea to read and research how to grow the plants.

The soil you chose is REALLY good...I'm liking it a lot in my grow. However, you still do need nutrients. You can either buy them online or at a hydroponic store near you. There are MANY different nutes people use and I really don't know whats better than what...I recommend for beginners the nutes called "Fox Farm Trio for soil". There have been a good amount of people here on grasscity getting good results with Fox Farm.
pH is important because it can make or break your grow...The way to measure water pH is by using some sort of pH meter or detector. They have cheap $5 eye dropper tests similar to what they use for aquariums or expensive digital meters that detect pH very well, but have to be calibrated before each use and stored in a special way. Your water pH should be between 5.5 - 6 for soil. I usually feed around 5.5 - 5.8 pH water and my plants love it.
Before measuring pH, you first get all the water you need for your grow. Then you mix in your nutrient. Once the nutrient is mixed well, test the pH and use a pH up or pH down correction solution to fix it.

Example: I have 7 plants, so I get about 1.2 liters of water in a container. Then I add the correct amount of nute and mix it into the feed water. Once I have all the nutes I need, I then test the pH of the water. Usually it comes out about 7.2-7.5 for me. I then get about 7 or 8 drops of pH down solution (acid) to lower the pH to 5.5-5.8. Then I mix it well, retest the pH and if I am happy with the current pH I feed my plants. I do this every day.

Now, you're only 20 days into the grow, so you're quite likely still in "vegetation". During vegging, you only use nutes that are used for vegging, not flowering. You can also use root enhancing nutes...but thats all up to you. WHen you flower, you need to change nutes to flowering nutes, not vegging.
Example: If you have Fox Farms Trio for soil, You only need to use "Grow Big" for vegging. Once your plant is at a good height to flower, you will change the light schedule from 24/0 or 18/6 to 12/12. Once you induce flowering, instead of feeding your plants with "Grow Big", use "Big Bloom" and "Tiger Bloom" instead. This will ensure your plant gets all they need to live through flowering.

Yellowing of the leaves can be a lot of things....
http://www.icmag.com/ic/showthread.php?t=11688&page=1&pp=15

 

No problem. Just make sure to read up or watch videos on how to grow..

Check out: the Cannabis Growing Bible, The IndoorOutdoor Medical Grower's Bible, Marijuana Buds For Less

Or if you wanna watch a really good growing movie/video, check out High Times Ultimate Grow- Growing Marijuana.