Grouping data in SSRS

To group data on a report you need the grouping pane. If it is not visible click grouping on the view tab.
In the grouping pane right click a group an select add before or after to specify where to add the group. This group dialog opens:

Set the name of the group in this dialog. After that the group expression can be specified. Just add grouping criterias. It is possible to group by data fields or by expressions.

When closing the dialog a new grouping pane is added to the tablix data region. it contains a row or column where group values can be displayed.