How to Photograph Silhouettes
Silhouettes often work best with strongly backlit scenes, so face into the light source (most usually the sun). This can often result in undesirable lens flare, so you may need to shield your lens with a lens hood and/or your hand. If your hand gets in the frame you can zoom out a little bit, and then crop your hand out of the resulting photograph so that there is no lens flare at all and you retain your original desired composition.
From an exposure perspective, silhouettes are actually easier to photograph than scenes where you want to show a full range of detail. If you do not care about shadow detail, you do not have to expose for it, and instead can just concentrate on making sure that the brighter parts of the scene are properly exposed. Most metering modes will try and "average" the light through the scene, which usually works great but not so much for silhouettes where such an exposure would render the shadows too bright. So use manual exposure, or dial back the exposure compensation a few stops so that the shadows are properly dark.
That being said, you may wish to capture all or most of the detail in the field (exposure bracketing if necessary) and then later decide how much of it to keep or throw away during post-processing. This approach is more flexible, though usually if I know I want a silhouette I keep it simple and do a single exposure.