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.