You can't to a pull up because of 2 reasons:
1) You're fat. Seriously now, all that extra fat means a lot cause you arms have to lift up extra weight. So get thinner, run/swim or something.
2) You upper body muscles are probably way below average, I'm at this conclusion because people who are fat generally do long hours on their computers and 0 exercise ipso facto, 0 muscles.
Combining the above two it is very clear that your journey to 1 pull up will be though, my recommendations:
Do lots of push up!
Do lots of sit ups/crunches!
Lose weight by running/swimming!
Don't eat fatty foods, AT ALL!
Be a vegetarian every week or so but take meat every 7 days or so.
Before you know it you will be doing 10 pull ups.