There are a wealth of online resources that should be taken advantage of when working with and/or learning Grasshopper.
First and foremost is http://www.grasshopper3d.com/, the project’s homepage. The site contains a very active forum, links to numerous tutorials and learning materials, and provides the most recent releases of the program itself. Also check out the always growing list of add-ons, some of which we’ll be using later in the term. If you have a GH question, this is the place to start.
Second: designreform.net. There are a bunch of fantastic grasshopper tutorials dating all the way back to when grasshopper was called explicit history. All tutorials are very high quality and do a great job explaining concepts as well as technique. This blog is a great resource as it offers tutorials across a wide cross section of parametric and rule based platforms.
Third: The Grasshopper Primer – Second Edition. This is a comprehensive document that describes, in detail, the basics of the interface all the way up to programming on top of GH. Although it was written for an older version of GH, most of the material is still very relevant. I don’t know of another document that provides as much information on as many topics. If you are looking for an in depth answer to a question, the Primer is often more helpful than the forums.