Quick Mask Method
If you want to learn how to make a silhouette out of a picture, but your subject blends into the background, or showcases a lot of light and dark areas -- which might make it harder for you to single out the subject with the Magic Select Tool or Magnetic Lasso -- the Quick Mask method is perfect for you. By using the Quick Mask mode, you can paint on the selection you would like with the brush tools. It requires a bit of a steady hand, and if you're lucky enough to have access to a graphic tablet, such as a Wacom tablet, it can also be a very quick method.
Click Quick Mask, which is located at the bottom of your tools toolbar.
Press D on your keyboard - this sets your foreground and backgrounds colors to black and white. For a list of other useful Photoshop shortcuts, you can check out my Photoshop Shortcut Cheat Sheet.
Click on your brush tool. Generally you're going to want a round, hard edged brush. Pick a brush that is small enough to do the outlining details of your subject and trace the inside edge of your subject, like so:
Now that you've got all of the outside detailed, fill in the inside. You can use a larger brush to make quick work of it. If you ever feel like you've gone over the outside edge too much, simply switch your foreground color to white, which works as "erase" in Quick Mask mode. This is also helpful for touching up the silhouette selection.
When you're happy with the way it looks, press the Quick Mask button again. You'll now see that everything except your subject is selected. Fix that by pressing SHIFT+CTRL+I (Mac users: SHIFT+CMD+I) to invert the selection.
Just as before, create a new layer and fill your selection with the color you desire.