I haven't done any silhouette pictures myself but I've seen this questioned asked a lot. I'm sorry if any of this is wrong, but only thing I have silhouette is my hand. Oh and someone please correct me if I'm wrong on any of this.
You need to meter the light itself first, be it sunset or other light. Use the "Av" mode set you aperture to whatever you want (ie: f/3.5) then press the shutter button down half way so it tells you the shutter speed. Say its says f/3.5 with 1/320 shutter. Now place your subject between you and this light source and in Manual Mode "M" on the dial. Put in the settings you got from the "Av" mode. You would want to be on the same path the persons shadow makes, to be sure you are using the person to block their area of light out. Since the subject is stopping the light hitting them they should show up as a black area. Now take your picture.
If they still are showing up with some color to them, then there may be to much light shinning on the side you want to be black (ie: another street light or whatever). Or use an even faster shutter speed to let even less light in, this will make the whole picture darker but it will help you get the silhouette effect.