Every MOC on Rebrickable has building instructions to help you re-create them yourself. These instructions vary in quality and ease of understanding. The designers put a huge amount of effort into creating the high quality instructions you can find on Rebrickable.
The official LEGO set instructions can be found on the set details pages, and are linked to external sites that do a good job of collecting them. Building instructions for LEGO sets that are currently in production can be found on the LEGO.com site itself. Older instructions can be found at several fan sites that have collected them over time: