What is the simplest way to be calmer, to relax, and to reduce stress?

Your breathing. Notice your breathing. Pay attention just to the air coming in and out through your nose. Or noticing the air arriving into your chest and exiting your chest, or even your belly. Just keep doing this will already help you feel calmer. Try it.

You can also breath out slowly and deeply, until all air is out, without straining. Longer exhale activates your parasympathetic nervious system, it will help you relax and reduce stress.

You may even make that exhale with a sound, like a sigh. It shall help you let out the stuck energy too.