The first time you sit down with your guitar, assuming you haven't had any instruction on how to do it, you'll need to understand how to hold it and have a basic understanding of hand positioning. Since the first chords you'll be learning will be open chords, you'll need to know how to hold the guitar with both your right and left hands as well has how to situate your fingers.
First of all, depending on whether you are right-handed or left-handed, you may want to hold the guitar one of two different ways. The standard way for right-handed players to hold the guitar is to use the right hand for strumming and the left hand for fingering and making chords. Many left-handed players play guitars this way as well, but some choose to purchase and use left-handed guitars that are designed so that one strums with the left hand and plays chords with the right. If you're left-handed, you must make the decision as to whether you'd like to play with a left-handed or right-handed guitar.
No matter what type of guitar you have, you'll need to know how to hold the guitar sitting down. Most players find it easiest to rest the guitar on the thigh aligned with the side of the body that strums the guitar. That is, if you play right-handed, rest the guitar on your right thigh. The guitar body should curve along the sides to rest comfortably there. Be sure the guitar doesn't lean against your chest or belly, but is perpendicular to the thigh. You should have good posture when playing to avoid leaning back too far or over forward too much as doing so makes it harder to move your fingers into the appropriate positions for many progressions.
While the body of the guitar is supported by your thigh, the other end--the neck--is supported by your other hand. This is the hand that forms chords and moves up and down along the fingerboard. In the beginning, you will be learning open chords, and you should be sure that your fingers on this hand curve around the other strings until they are situated in the appropriate position on the fingerboard to play a chord correctly. That is, they don't rest flat against other strings that they are not supposed to touch according to the chord fingering. This will create a dull, muted sound that results in only a few of the notes of a chord being played correctly. While this may cause some discomfort at first, it's necessary to develop this positioning of the hand and fingers so that you can further develop as a guitarist. When playing these chords, the bottom of your palm should be pressing against the back of the neck to help your fingers close and press down upon the appropriate strings on the fingerboard. With practice, your ability to play chords with this positioning of the hand and fingers will improve greatly.