Use the adjective preparatory when you're getting ready for something. If you spend the day cleaning your house before a big party, the work you're doing is preparatory.
You can call something preparatory when it helps you prepare. A summer school class that lays the groundwork for a calculus course in the fall is preparatory, and so are a coach's words of advice before the start of a football game. High schools that specifically ready students for college work are sometimes called "preparatory schools," or "prep schools" for short. The Latin root is praeparationem, "a making ready."