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Vectors can be easily rescaled without losing image quality. If you're using the silhouette professionally, or just want a more versatile final image, this is the way to go. Adobe Illustrator (AI) uses vectors almost exclusively. If using AI, skip the quick route and use this method. If you've duplicated the layer already to ensure that you don't ruin the original image, you should be fine. Just make sure you have a second layer to work with above your original image. To create a new layer, press Cmd+ Shift+N or Ctrl+ Shift+N Choose the Pen Tool (P) from the toolbar. In the menu that appears at the top of Photoshop, find the small drop-down menu that says "Path." It should be to the top right of the screen, but only when the Pen Tool is on. Change this menu to say "Shape." Taking your time, trace out the shape of your silhouette. To make things easier to see, lower the opacity of the new layer you're working on by changing "Opacity" from the top of the layers panel. Once you return to the starting point your dots should disappear and the shape will form in front of you. Raise the opacity back up to 100% to see your silhouette. Once this shape is made, you can do anything with it that you might normally. If you want to isolate it so your image is only the silhouette, either delete the layers below it or click and drag the image to a new Photoshop document.
Use vector silhouettes if you want to adjust, grow, shrink or export your silhouettes without losing quality. Create a new layer above your original image. Select the image you want silhouetted with the Pen Tool. Use the Pen Tool to trace out your entire silhouette. Connect the dots back to finish your silhouette. Click and drag the silhouette into its own image, to Illustrator, or leave it to finish your silhouette.