Always work out at your own level. If a standard pullup is too challenging for you, or you can’t get a decent number of reps that way, use one of the intermediate levels (they go up in difficulty from 1 to 3). It’s OK to stretch yourself, but there are no bonus points for hurting yourself.
Grab the bar at a little more than shoulder width. Use an overhand grip for pullups and an underhand grip for chinups.
Intermediate Level 1
Put a box or chair under the pullup bar. Pull up keeping both feet on the box. Don’t push with your legs.
Intermediate Level 2
Put the box or chair slightly to the side. Pull up keeping one foot on the box. Don’t push with your legs.
Intermediate Level 3
From a level 2 pullup, lift your feet off the box, hold for 2 seconds, then gradually lower yourself down.