But a few things that are fun about the silhouette is that it forces you to simplify. The subject is simplified to an outline. The lighting is simplified to simply underexpose your subject.
Put your camera in M mode, if it's not already. Let's go with ISO 200, f4 and shutter speed of 8000.
Get your subject between you and the sun. And you may find the focusing is a little difficult. The focus on the camera works best with contrast. So if it is only seeing the dark (underexposed) subject, it can struggle to focus. Manual focus may be necessary.
Play around with the shutter speed, maybe go down to 4000.
Even play around with the f stop. If you hit an fstop of f16 you could get subject and your background in focus. Remember to slow down your shutter if you do that.
Another important note is to keep the background fairly well lit. You can really lose your subject, as in my example here, if the background has a similar light (or dark) value.
ISO 200, f4 at 1/8000 95mm
Here is another attempt but with better composition with my background... or as Cliff likes to say, juxtaposing the subject with the background.
ISO 200 f4 at 1/4000 116mm
Remember that your background really speaks in a silhouette.
Happy shooting... would love to see some links in the comments to you silhouette shots.
PIC of the Day: This shot at ISO 100, f5.6 at 1/2000 300mm