Get yourself an armless chair, and take a seat. You should be sitting comfortably, with your back against the back of the chair. Slouching significantly is a no-no; you’ll not only end up with a sore back, you’ll develop bad habits on the guitar.
Now, pick up your guitar, and hold it so the back of the body of the instrument comes in contact with your stomach/chest, and the bottom of the neck runs parallel to the floor. The thickest string on the guitar should be the closest to your face, while the thinnest should be closest to the floor. If this isn’t the case, turn the guitar the in other direction. Typically, a right-handed person will hold the guitar so the headstock points to the left, whereas a left-handed person will hold the guitar so the headstock points to the right. (NOTE: to play the guitar as a lefty would, you will need a left-handed guitar.)
When playing the guitar sitting down, the body of the guitar will rest on one of your legs. In most styles of guitar playing, the guitar will rest on the leg farthest away from the headstock. This means, a person playing the guitar in a right-handed fashion will typically rest the guitar on his/her right leg, while someone playing the guitar in a lefty manner will rest it on their left leg. (NOTE: proper classical guitarist technique dictates the exact OPPOSITE of the above, but for this lesson, let’s stick to our initial explanation)
Next, concentrate on your “fretting hand” (the hand closest to the neck of the guitar, when sitting in proper position). The thumb of your fretting hand should rest behind the neck of the guitar, with your fingers in a slightly curled position, poised above the strings. It is extremely important to keep these fingers curled at the knuckles, except when specifically instructed not to do so.