Launch QuickTime by clicking its icon in the dock or from Applications:Utilities.
Select Open File.. from the File Menu
Select your video using the file finder dialog then click the Open button
Select Show Movie Properties from the Window Menu
Select Video Track from the list of tracks
Click one of the rotate icons that appears below
Notice your video clip has now been rotated...
Select Save As from the File Menu.
Make sure Save as a self-contained movie is selected, enter a name for the rotated clip and click the Save button.