Now when you have familiarized yourself with the parts of guitar, lets begin with the real stuff now.
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.
I will suggest you that as of now follow the steps mentioned here to hold the guitar and later once you become comfortable, you can sit as per your convenience, else you may develop a bad habits to play guitar. As good guitar playing sometimes depends a lot on the posture of the guitarist as well.
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 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 and 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.
* First put it at 90 degree from ground, in your lap.
* Then tilt the guitar towards you to about 30 degree
* Then lift the guitar from the head of the guitar upwards at around 30 degree again.
You are not supposed to bend towards the fret board. The fret board should be visible to you with the position you are sitting in. If its not visible then tilt the guitar more towards you to few more degrees you should not bend and bow your head to see the fret board. You should be in ease.
Now 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 should never see the face of the fret board, which means, that the thumb should remain at the back of the fret board, invisible to the fret board as much as possible.
The reason behind it is that if you become a regular guitar player, you can suffer from pain in your neck especially for people with more fat and flesh. Else also, you will find it later when you start playing good guitar, that this thumb rule basically help to play better guitar.
It is extremely important to keep the fingers curled at the knuckles, except when specifically instructed not to do so.
You should not put your left elbow to rest on your thigh or anywhere playing guitar. It should be floating.