In this step, we'll adjust the values in the image to make it black&white instead of gray&gray.
First, make sure Layer 1 is still highlighted. Next, go to Image>Adjustments>Brightness/Contrast. The Brightness slider changes how light or dark the image is. At -100%, the image is completely dark, and at 100% totally bright. The Contrast slider changes how much of a difference there is between the brightest bright and the darkest dark. At -100%, the image is middle gray (there's no difference between the light and dark parts), and at 100% pure black and white with no grays at all. We want levels that will make the lines stronger and darker, while making the paper part of the image whiter. Adjust the brightness value to 35% and the contrast to 53% - that's a good estimate for most black pen on white paper (though it depends also on your scanner). Click OK, and you've got yourself a black and white drawing to start with. The next step is color!