Pull up with assitance: One option is to use an assited dip and pull up machine, these are quite common in gyms. If you do not know how to use one, ask. I can't stress enough how important it is to get proper instruction.
Okay so you can almost do a pull up, then what: get a partner to train with and have them spot you by assiting your progress by gently pushing you up while you pull, usually done with their hands just above your low back. If you don't have a training partner, you can grab an exercise band, loop the handles over the end on the pull up bar making a hanging stirrup for your feet. Step into the stirup and have the band assist you as you pull.