Bok choy is an Asin cabbage variety. It grows in sandy soil, so you need to make sure to clean any grit from between the leaves.
Depending on how you are preparing the bok choy will depend on how you clean it.
If you want to cook with individual leaves: chop the bottom of the bok choy head to release all the leaves. Clean leaves under running water until any visible dirt is removed.
If you want to half or quarter the bok choy: make your cuts keeping the stem intact. Then rinse in a bowl of cold water, shaking the cabbage around to lose any dirt. Let drain.
I like grilling bok choy or sautéed with garlic and oil.