The below is for Photoshop Elements 8 (for Mac), although I assume the steps are largely similar in other versions of Photoshop.  The below assumes that in the end, you want a jpeg for a web page.

1. Load your photo in Photoshop
2. In the Layers pane, double click on the single layer that’s there corresponding to your photo.  When the dialog pops up, just click Ok to name your layer Layer 0.
3. In the Layers pane (bottom right pane), in the bottom toolbar, click on the little circle icon (‘create new fill or adjustment layer’)
4. Pick solid color.
5. Enter the hex color of the background of the webpage that you will eventually put your photo on.
6. In the Layers pane, drag your photo layer (Layer 0) to the top, so that it appear above your new Color Fill layer. If this doesn’t work, did you check to see that you followed Step 2?
7. Go to Image->Resize->Canvas Size in the top menu.
8. In New Size, choose pixels.  Add 100 pixels to width and height.  Click the top-left arrow under Anchor.  Click Ok.
9. In the Layers panel, select your photo layer (Layer 0)
10. In the Effects panel, choose Layer Styles (the little overlapping windows icon).  Then from the dropdown menu to the right, select Drop Shadows.  Select the drop shadow effect you want (probably low or high).  Click Apply.
11. Your drop shadow should appear.  If you want to adjust the effect, double click on the little fx icon in your layer’s entry in the Layers panel to open the settings dialog for that effect.
12. If you mess up, right click on your layer’s entry and Clear Layer Style.
13. Save your psd file.
14. Go to File->Save for Web, pick your preferred jpeg quality and image size, apply, and click Ok.

Note: if you want a png, then instead of creating a new fill layer in Step 6, you would add a normal layer, and make sure in Step 14 you save the png with transparency enabled.)