This practice of jotting and scribbling is called freewriting--that is, writing without rules. If you find yourself searching for a writing topic, start by jotting down the first thoughts that come to mind, no matter how trivial or disconnected they may appear. If you already have at least a general idea of what you will be writing about, put down your first thoughts on that subject.
How to Freewrite
For five minutes, write non-stop: don't lift your fingers from the keyboard or your pen from the page.