Hello grace. I'd say there's two must, even if you're not a total beginner. The very first is The Basics:
https://cgcookie.com/courses/blender-basics-an-introduction-to-blender-3d-4-5-lts
Then Fundamentals of Mesh Modeling:
https://cgcookie.com/courses/core-fundamentals-mesh-modeling
After those first two you have a good idea around a mesh and you can jump to the 3D printing courses:
https://cgcookie.com/courses/make-it-3d-printing-with-blender
https://cgcookie.com/courses/printlab-your-guide-to-3d-printing-with-blender-and-a-prusa-printer