One way to ground yourself and bring yourself to a state of calm is by practicing deep breathing exercises. Focusing on your breathing (something that all people do) can help you keep negative thoughts at bay and prevent worrying because you are focusing on something else with deep concentration. The most important thing with any relaxation exercise is to practice it regularly so that it becomes “routine.” In the beginning, it may induce a little stress because it’s new (not routine). The goal is to make it feel so “normal” to you that your body physiologically responds by relaxing as soon as you start the exercise. Remember: don’t do these exercises if you have medical reasons not to, and consult your doctor first if you have limitations.
You can do deep breathing anywhere without being too obvious—standing in line somewhere, sitting in class, etc. There are some specific postures that are recommended, though. Don’t do it while lying down (unless you are trying to go to sleep).
Your mind will want to wander. Just keep bringing your focus back to your breathing and counting. This takes practice to work and calm you. Try it for a few minutes at a time and slowly build up to longer periods. The more comfortable you become with deep breathing, the more it will help relax and calm you.