When you play in a sitting position, the guitar can rest on your left or right leg. If you are right handed, it is most common to have the guitar on your right leg, but the choice is yours.
There are a few things you should remember when you are sitting with the guitar. Have a good sitting position with your back straight. Bad posture can lead to poor technique, while allowing for a bad back in time.
The next thing you should remember is to keep the guitar close to your body, with your strumming arm. Avoid that the guitar slides down your leg. Many beginners lets the guitar slide down the leg so they can better see the strings and. Try not to fall into this trap.
Try placing the guitar on your left and right leg, and see what works best for you.
You can also choose to stand up when you play. This requires that you have a strap on your guitar. Adjust the strap so the strings are in front of your navel to have the best posture.