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).