Ah! I see! Easy to confuse.
Not going to lie to you; pull ups can be a bitch for many people. Quite a few people can barely do 1, (One proper one that is, as in arms starting from straight and chin over the bar) if that. If you've never done it before prepare for pain! It works pretty much works nearly all the upper body muscles.
Find yourself a bar to pull yourself up on. If you can't manage 1 pullup start by standing on a chair/jumping to the top and then lower yourself
slowly until your arms are straight, stand up again on the chair and repeat.
Pull ups predominantley work the Latissimus dorsi (Or Lats, those large muscles in the middle of your back.) If you can't find a bar try
doing some rows at quite a
high weight; your test is going to have you lift your body weight, so make it count.
Be sure to work other parts of your upper body (including abdominals) as the pull up also utilises this.
Random note: Chin ups are when you have your palms facing towards you, and pull ups have your palms facing away. Chin ups make use more of biceps whilst pull ups use the lats. Just making sure we have the correct terminology here!