Hi there,
Pull ups are a great challenge. What have you currently been doing to increase strength, and do you feel you have been making progress? Before I started weight trianing I was VERY weak. A pull up was well out of reach. I can now do 9 unassisted pull ups. If I can do 9, I think you can do one.
Are you doing negetives? This is where you grab the bar and gives yourself a push up, then lower yourself very slowly. You could try starting this way and as you progress give yourself less and less of a push until you don't need a push at all.
Please let me know if that didn't make sense or if I can help more.