Rarely do you know what you want to sketch before you should start. I don't remember who said this, but my advice is:
Begin anywhere.
Your goal is not to have perfect sketches, but to use sketching as a thinking tool. As you practice this, you will realize how sketching catalyzes your thought process, and you will get better and better over time. As you improve, you will become more confident and relaxed, and your sketches will naturally become more expressive and beautiful. My first "AHA" form breakthrough came with sketching for a trash can project. I had done about 4-5 pages over 2 hours, and was getting tired. But, I pushed through and on page 5 or 6 after sketching trash cans enough, the simple idea came to me to improve the function through the form.
In my experience, your best idea usually comes right at the beginning of your process, or at the end. You should never stop after your first good idea, though, because it may lead you to better ideas. The way to get to the best idea is to have many ideas.