If you want a silhouette, you can spot meter on the area you want as a silhouette and underexpose that area by 2 stops. That requires shooting in manual mode, or you could shoot with your EV set to -2 and use the exposure lock after metering on the silhouetted area. -2EV is a starting point. Your camera's monitor probably won't show your a truly accurate image so check the histogram, which should be shifted far to the left with a spike on the far left. If -2EV leaves you with more detail than you want, either underexpose more, or make your corrections in Photoshop or another program. My preference would be to shoot RAW since you may end up underexposing the background to produce your silhouette.
Richard
Great description. I've always spot meterd or center metered off the sky, and that underexposes the subject... but I like the -2 EV better.