There are two basic ways that you can choose to hold the guitar when you are sitting down. You can hold it over your right leg or your left leg. If you are right handed the more common choice is the right leg but the choice is yours.
There are a couple of things that you should remember when holding the guitar. One thing to remember is to have good posture and not slouch too much. Poor posture can lead to poor guitar technique and it can be pretty bad for your back too. The next thing to remember is to hold the guitar close to your body with your strumming arm. Don’t let the guitar slide down your leg. A lot of beginners let the guitar kind of slide down their leg so that they can see what is going on better. Try not to fall into that trap.
Watch the guitar lesson video, experiment with holding the guitar over each leg and see which one works best for you. Once you are done with this lesson you can move on to the next lesson in the Quick-Start Beginner Series