Basic (start here!)
Pull ups are one of the single best exercises you can do, but like most things that are worthwhile, they can be difficult.
Here are several options to get you up to speed, just sub for regular pullups till you are able to do them.
Generally 3 subs = 1 regular pull-up
Put a towel or rope over the bar and pull yourself on a horizontal plane, changing the angle varies the resistance.
Put a box under the bar and assist yourself with your feet
Jump up to get momentum and pull as much as you can
Jump or step up using a chair and hold as long as you can
Jump or step up using a chair and slowly lower yourself down
Mix and match these during your workout using a variety of grips (e.g. palms away, palms toward you). Allow yourself to progress slowly and before you know it you will be doing regular pull-ups!
When going up a level, just post type of pull-up substitutions used.