序 (Kanji for first part / preface)

Onyomi

JO	Finding your dick is the first part of Jacking Off.

You have to go in the first part of the cave before you go into the depths.

じょ	the first part - also means the order. as in, 序に xxx　（in　order of XXX) LAZY ☆☆☆☆☆

序文(じょぶん)

Preface ★☆☆☆☆
序 (first part / preface) + 文 (culture/ sentence) = 序文 (Preface)

Preface or introduction.

順序(じゅんじょ)

sequence of steps ☆☆☆☆☆
順 (sequence) + 序 (first part / preface) = 順序 (sequence of steps)

the sequence of steps in a proceedure - the order of books arranged by a certain system, (i.e. Dewey Decimal).