I know this blog is mainly about kettlebell training, but if you look back in the archives, I try to not limit the content. That is especially true for training methods that are particularly effective. Pullups are one of those exercises that everyone SHOULD be able to perform, but most can not do. Show me any strong human, and I guarantee they can bang out at least a couple of pullups. Even the 300 lbs heavyweights of strongman, powerlifting, and olympic weightlifting can bang out a solid 10 reps, so I don't wanna hear crap about your weight.
Performing a proper pullup is not as intuitive as you might think. If you just "pull up", overtime, you will create an overdevelopment of your lats and biceps, which will lead to shoulder problems. You first need to depress the shoulders during a dead hang, then while keeping them depressed, you drive your elbows down and chest up. Here is a good video of Dr. Horwitz breaking down proper pull up technique.
So what do you do if you can't even do 1 pullup? Well, you have several options. First, you can start with jumping up to the bar, and trying to hold yourself close to the bar for up to 20 seconds. Next step is to try eccentric pullups, which means jumping up to the bar, and then lowering yourself down as slow as possible for several reps in a row. The third step is to attach a band to your pullup bar, then wrap the bottom of the band about your knees. This will subtract weight from you, more so at the bottom of the rep, and giving less help at the top of the rep. Feel free to give any of these training methods a try, but above all else, start doing pullups in your weekly routines.