Did you know that if you can't do pull ups yet (you will one day!) you can still do lat pull downs? Make sure you use correct form every time you do it. You want to aim to have your hands just outside of shoulder width. If you use too narrow of a grip you'll shift the focus to your forearms. To target the muscles in your back, be sure to lean backwards lightly. The target point of the lat pull down is your upper chest. Imagine driving your elbows down into your front pockets as you are seated. Make sure you are in control in every phase of the movement, both as you pull the bar down and as you let it back up again.
Enjoy!