You don’t need to take the classes in any specific order; each is designed to stand on its own. That said, each class page includes recommended class pairings to round out your understanding of the subject matter, and if you are new to human design we recommend buying the classes in the order they are presented:

Intro to Human Design, Types, Strategies, Not-Self & Signature, Authorities, Open & Undefined Centers, Defined Centers, Profiles, Definitions, Channels, How to Read a Chart, Variables.