I think you might want to define the user requirement in a bit more detail, or have the user provide it to you.

Are they looking to:

1. sort the output before it is rendered

2. sort after the output/report is rendered

Option 1: A drop down parameter is added where they select the column they want to sort on and then an additional one maybe giving the option of DESC or ASC. Using this method would involve expressions as the example "pietlinden" provided.

Option 2: Is (again as already mentioned) using the tablix properties on each column enabling sorting. This would be the easiest to implement and maintain. As well gives the user more control over sorting of multiple columns.