I have coached dozens of men how to bang out pull-ups and even a handful of dedicated women. A single, unassisted, USMC-style pull-up is a very high benchmark of physical fitness for the ladies, but it is completely attainable. In fact, if you have a BMI of less than 25 and no serious shoulder injuries, follow my advice and you will do your first pull-up in under 2 weeks.
1. Buy an Iron Gym. Do not skip this step. The damn thing costs $20 and mine has held up (with mild modifications) for over 20,000 reps. Seriously, this is bang-for-your-buck.
2. Install it in a doorframe that you walk by frequently.
3. When you walk by the door, jump up on the bar to the top position of the pull-up and bend your knees.
4. Lower yourself down as slowly as possible until your arms are locked out.
5. Repeat steps 3-4 every time you walk by the door!
I'll bet that before you can say "Latissimus Dorsi" you will be able to walk by that door and do steps 3 and 4 in reverse. Simply put, every person who has asked me how to do their first pull-up and followed these five steps has succeeded. Every person who asked and did not follow these steps, failed.