If you're unable to do a proper pullup, or if you can, but just can't get more than 1 or 2 and need to do more, there are ways to assist them.
Assisted pullup machine. Machines like the Stairmaster Gravitron can be found in many commercial gyms. They provide a mechanical assist to your pullup, allow you to do some of the work while the machine provides the rest.
Partner Pullups. You can have a partner stand behind you and grasp your body or legs and give you an assist up. This can be a full assist - holding you throughout the movement, so you get both an assist up and steadying on the way down. It can be a partial assist - a boost up, and then a release so you can finish the rep yourself. These are tricky, though, because you can't really measure the amount of assistance with much precision.
Band-Assisted Pullups. Using a long (usually 41") band, loop one end over the pullup bar and use the other end as a stirrup for either one or both of your legs. Either way, the amount of assistance is the same. While it's still hard to gauge the poundage of the assist (band resistance varies as it is stretched or relaxed), you can move from a thicker band to a thinner band as your strength improves.
If you're struggling with pullups, give those a try. The last - band-assisted - seems to be the most productive, as the assistance is less as your get closer to the top, forcing you to do more work. All of them should be used only until you can get a handful of reps, and then you're better off working up the numbers by doing full pullups.
If you can already do a few pullups but want to get more, this article (not W/F S) might help.