Icons are a very specific type of image. They are simplified and usually quite small, so you will need to draw out your idea out several times to cook the image down to its essential parts.
As you can see from the images, icons have to be able to “work” at very small sizes. When I say work, I mean they need to be legible and readable. They need to be obvious in what they are and they need to be clearly rendered. What looks good at one size may not work at a smaller size.
Here is a great walk through of developing an icon from a pencil sketch.
The way I start an icon sketch is I break the object down into the simplest shapes possible: squares, circles, triangles, or rectangles. Then I add details from there.