I'm quite new to scripting in Grasshopper. Is there a manual where I can find all types of variables that I can define in GH. Like Planes, Datatrees, Paths, etc?
Is it in the SDK? If so is there a manual/guide how to use his sdk - I don't really knoe where to start searching.
Grasshopper is a non-encrypted and non-obfuscated .NET library, so it automatically has an SDK. You should download Grasshopper 0.7.X as the SDK has changed quite a bit. For one, 0.7 is build on RhinoCommon.dll[1] while Grasshopper 0.6 is build on Rhino_DotNet.dll
Once you have GH 0.7 running, you can download the SDK helpfile from the Help menu.