Pull-ups are the hardest body weight exercise we can do and probably the most hated of the exercises.
They are hard to do and many people don't want to or refuse to do them even knowing it can build a really strong back.
You are missing out on a great way to get stronger.
Most people can easily do push-ups but when you hang on the bar for pull-ups your full body weight comes into play.
A lot of guys would rather use a lat pull down machine than do pull-ups for back development which is good but still does not match pulling your weight up to a bar.
If you have trouble doing pull-ups just start doing them one at a time.
Do one and take a break and come back for another, this is a lower stress way to get stronger.
As time goes by you can do two, then three and on up.
Not able to do one pull-up yet, no sweat.
Put a chair under the bar and stand on it.
Holding onto the bar, step off the chair and lower yourself down.
This is called a negative and is a great way to get stronger, do these in a set of 6 to 10 reps maybe 3 times a week.
Eventually try to pull yourself up to the bar.
You could also have a partner help lift you up as you pull.
You should do most of the work but your partner can get you past a sticking point.
Any of the above methods would be a good way to get started, once you get stronger keep it up and you will see and feel the strength difference.