What are some exercises in order to get strong to do a pull-up?
You can start by just hanging from the pull up bar in the down position.
Be careful that your shoulders don't get stretched out of their socket though! (if that makes sense)
After you can do like 2 sets about one minute, you can move on to the next step, which can be either jumping pull ups or negative pull ups.
You basically use a chair or similar to help your self to the up position, using your legs for negative pull ups.
For jumping pull ups, you grab the bar, and use your feet to jump to the up position.
Either way works great, but be sure to squeeze your lats for a second on the up position, and then SLOWLY lower yourself.
Once you are strong enough to do like 2 sets of 10 from either, you can start with normal pull ups.
Just stand under the bar, aim for a number, and don't leave until you complete it.
Start with like 5, move up to 10, 15 ... up to 30 one day!
Also keep in mind, chin ups are easier then pull ups, because you use your biceps more.