Sometimes, even if thoughts are running through your head like crazy, the mere act of slowing down your breathing can make you feel more relaxed.
Sit upright and relax. Close your eyes and focus on keeping your body still. Then, start to slow down your breathing. Let thoughts come and go, don't try to stop any thoughts. Just focus on breathing slowly. If it helps, you can even count to 3 while inhaling and exhaling, to make sure you are breathing slowly. If you maintain this for a few minutes, you should find yourself feeling a little calmer and more relaxed, as you go about your day.
Source: Meditation for Beginners course by Ken Wells