To keep in view a row with column headings for a table with no row group 

In Design view, select the tablix data region. The Grouping pane displays row and column groups.

On right side of the Grouping pane, click the down arrow, and then click Advanced. The Grouping pane displays static and dynamic tablix members for each group. 

In the Row Groups pane, click the static tablix member that you want to control. When you select a tablix member, the corresponding cell on the design surface is selected if possible. Not all tablix members have a corresponding visual element. The Properties pane displays the properties for the selected tablix member.

Set the FixedData property to True. 

Preview the report. The column headings stay in view as you scroll down the report.