You have some options.
1. Start with a low bar/high bench. Position yourself with your chin above the bar, step off the bench, lower yourself slowly, step back on the bench, position yourself again and repeat.
2. Use a thick resistance band. You loop the band around the bar, put your foot in the bottom of the loop, and then do pull ups by using the band to offset part of your weight. You might start with three sets of 10 reps. If you can't do 10 reps, get a thicker resistance band to support more of your weight. If you can do a lot more than 10 reps, get a thinner band. Gradually work down until you are using no band at all.
3. Use a thick resistance band. Stretch it between the brackets that hold the barbell. Grip the bar, step off the bench onto the band, and sink down onto the band. Then, do your pull ups. You can make it easier by using two bands or by moving your feet farther apart on the band, which effectively increases the amount of weight the band offsets.