Selecting a trek is probably the most difficult part. Many things has to be balanced including the number of days and the amount of pain one is willing to take. Also it's important to make a decision that whether you want to go with a well planned group where everything is laid out minute by minute to a more adhoc approach where you find the way as you go. Be careful that the first one may make it too artificial and at other end where you would just spent most of the time finding the way.
This is an exhaustive list so pick the things as you need. Also this list assumes the trek to a cold place. To a hot place the things might change.
21. Your photo id proof.