Drop & Roll into a corner.  keep rolling.  Don’t stay still.  Modern handguns are powerful enough to plow through wooden desks, chairs, sofas, etc.  Get concealed, roll (or crawl) into a protected area like a corner. But be aware:  drywall can be punched through.  A bullet will go through drywall like tissue paper.

After you get concealed or against a wall, search for a way out.  Then get out if it is safe.  According to what I just read about Ft. Hood, several people saw the shooter stop to reload and *then* ran out the door to get away.  That’s best. Nice thinking people.  No panic, just careful planning on no notice.