You can format a text box by right clicking it and selecting Textbox properties. Then in the format/text pane (I can't remember what it called, and i don't have SSRS on this machine), you can select your format you desire, or enter a custom one (such as dd/MM/yyyy, although this should be a format in the dates selection).

Alternatively, you can select the textbox and hit F4. Then in the properties pane there is a field called "format", where you can enter your customer format. When doing it this way, you can highlight multiple textboxes, and apply the same format across the all of them. You can't do this in the dialog box.

To do this, you will need to ensure that the T-SQL statement returns the date in a date, or datetime field type, or the format won't work (SSRS won't understand how to change the look of a string formatted in a date like format).

Hope that helps.