Well the stairways here are always pretty good, this one should have everything you need. http://www.sqlservercentral.com/articles/Stairway+Series/72649/
I always learn the best by doing. And for SSRS grouping, I always start by using the Report Wizard. During this process it gives you the ability to select the groups in your report. Build a sample report that has a few hierarchical groupings in it so that you can experiment. Put at least two columns in the groups section and the rest in the detail. During the wizard there is a checkbox to "Allow Drill Down", or something like that. And then finish the build.
This will correctly set up the report to hide each grouping (toggling visibility based on the group above it). Right click any of the groups (including the detail section) and go to Group Properties. There is a section for visibility where you can see how the toggling is configured, the rest should explain itself.