To be able to do a pull up it's vital to have scapular contraction. In other words, your shoulder blades need to be pulled down into place to get the maximum squeeze through your lats. We're going to use a lat pull down and do scapular contractions.
Place your hands about shoulder width apart, brace your core, and then relax your shoulder blades up toward your ears. From there contact your shoulder blades down using your lats, hold for about a second, and then relax back up again. You can repeat this for 5-10 repetitions. This exercise will essentially help you to improve your back strength which is essential for doing pull ups.
Have a great day!