Creating silhouette images is one of the easiest in photography, and often produces interesting pictures. The trick is to control the placement of the negative space in the composition, and to make sure your flash is turned off.
For example, in this aquarium shot, I had to wait a while until I had two people positioned on the left side. The process went something like this: 1) saw the potential for a good shot, 2) grabbed a position that had the right composition, 3) checked my camera settings, and 4) waited...
Once the elements come together, the actual picture taking is simple. The camera will read the large bright area and usually ignore the silhouetted subjects. So your photograph should be good to go without too much post production fooling around. Just make sure you're not in "auto everything" mode and that your flash is turned off. I think regular "program" mode works great for these types of compositions.
I like the graphical nature of silhouettes. And when these types of images are mixed in with your "regular" photos, they'll certainly grab the attention of the viewer's eye.
You can find more photo tips and "photography how tos" on my Pinterest page.