【Preliminary】 ,  【introductory】 ,  【preparatory】 ,  【prefatory】  describe something that serves to make ready the way for something else.

【Preliminary】 suggests reference to what must be done or made ready or acquired before entrance into some definitive state or activity becomes possible.

【Introductory】  usually implies reference to the first steps in a process and therefore seldom applies to what is a prerequisite, as does  【preliminary】 , but rather to what sets an action, a work, or a process going.

【Preparatory】  comes close to  【preliminary】 in meaning, but it throws the emphasis upon matters that should be attended to in order to make a person or thing ready for what ensues or may ensue.

【Prefatory】  usually suggests not absolute need of preparation but a desire on the part of someone to prepare others (as for reading, for hearing, for action, or for understanding).