I teach Zen, which basically means that I help people sit still and focus.
Most people have a hard time with this, and it’s becoming worse with your generation. The options for distracting oneself have become so numerous and readily available that people commonly reach adulthood without ever having had to develop steady, undivided attention. But you’ve already reached an important first benchmark: you’ve turned your attention inward and recognized the unsteadiness of your mind. That’s a good beginning.
The next step is to deepen that introspection. Become a student of what your mind is doing, how your attention functions. Even as your mind is jumping all over the place—and your body is echoing that unsteadiness—develop a non-judgmental awareness of all that mental and physical fidgeting. You’ll begin to acquire a habit of knowing what your attention is up to, moment by moment. Think of this as meta-attention: attention to your attention. This alone will go a long way toward stilling things down.
To go further, you can deliberately put yourself in a situation where you exercise the skill of stilllng your body and mind. That’s what meditation is. You’ll probably find it frustrating, difficult and confining. Your mind will rebel against the discipline. Chances are you’ll drop it, because it’s so contrary to your fidgeting habit. But if you manage to stick with it, it’ll eventually lead to the kind of steadiness your hoping for.