There is nothing in the world quite like performing your first unassisted pull up with your body weight. The pull up has become my favorite exercise but I really had a hard time starting out, being as I am such a heavy-set guy. What ultimately did it for me was performing controlled negatives and machine supported pull ups until I could finally do my first pull up after several months. Now my goal is to do 5-10 unassisted pull ups in a row with proper form. I'm hoping in a year that I will be able to accomplish this. In the meantime I will continue to do 10 sets of 2-3 pull ups so as not to overwork my muscles or risk injury.
If you're a beginner and can't do pull ups with your body weight, the next best thing is to use an assisted pull up machine.
This exercise will have the largest impact by far. Remember: an assisted pull up is still a pull up. The next best exercise is the lat pull, using either a machine or cables.
Of course, you will also want to work your biceps on a regular basis.
Let me also add what is called an "eccentric" pull up. This is where you use a step stool to get you to the top of the bar and then you slowly lower yourself back down again.
A variation of this is called the "jumping" pull up where you actually jump maybe half way up to the bar and then raise yourself the rest of the way.
All of these variations are helpful because all of them involve a PULLING motion. The idea is to build pull up strength by doing easier variations of the exercise. When I was at zero pull ups, I couldn't do anything to progress until I had become consistent with exercises like dumbbell curls and lat pull downs. But the main way I got stronger was to start at the top of the bar and then use my muscles to gradually lower myself down. Once I could do this easily that's when I could actually train for pull ups. Doing only negative pull ups for weeks and months strengthened me enough to do my first unassisted pull up.
So to summarize, if you want to do pull ups, I think there are actually good reasons to do other exercises first. To me, it makes perfect sense to work the muscles involved separately in order to strengthen them before performing the main movement you're working towards using these muscles combined. Those separate exercises can help you find specific weak points so that you can focus more on strengthening them first, which you can't necessarily do with easier variants of the pull ups themselves. Once you have built up strength in your arms and back, my best advice to start doing pull ups is to hang on the bar every time you're at gym as long as you can do so comfortably. Then, if you can, jump up and lower yourself down slowly.
Pull ups are one of the most widely used exercises out there and for good reason. They're not only convenient to do but they are very effective at growing and strengthening your mid and upper back as well as your arms. As you are pulling, simply think about pulling yourself up by leading with your chest and trying to get your upper chest to the bar. Don't lose your form at the top of the exercise, and be sure to lower yourself back down slowly and purposefully.
Hopefully you are able to see that although the pull up is a killer exercise for many of us, executing it in the right manner is not beyond the ability of anyone who is regularly engaging in the art and science of bodybuilding.
Well, hopefully this will help you to get your first pull up!