I was in the same boat before this past summer. I hadn't done a pull up in my life. It took me about 3 months of working at it to finally do one unassisted. Band assisted pull ups were the biggest help--I think having to do the full motion was important. Also, the finish at the top was the hardest part for me and the bands forced me to still work at those since the tension lessens at the top. Like everyone else said, negatives and holds were also important. Occasionally I'd also throw in jumping pull ups--my bar is low enough that i can push off with my toes. Those were good finishers to a pull up session. Just keep at it.