Resizing images is one of the quickest things you can do in Photoshop. If you're uploading photos online or emailing them to friends and family, the massive pictures that come off your digital camera (or photo cd) are going to take forever to send, and may even not by received because of their size.

To start, load photoshop and open the image file. Now that you've gotten the image loaded, Go to the Image menu at the top, and click Image Size. Input the size that you want the image to scale down to, and hit the OK button.

You should choose your image size based on where you plan on using the image. If you're sending it in an email, it would be fine to have an image around 1024x768 (if it's landscape oriented) or 768x1024 (for portraits). Web images generally are a bit smaller, so you can show them on a webpage without it running past the edges of the browser or messing with the navigational elements on the page. Most of the web images I save are around 640x480 or so.

When you've selected the size and hit okay, simply go to the File menu and hit Save As. If you just hit save, it'll overwrite your source image and you'll lose the higher resolution version. When I make copies of images, I usually store them as filename_resolution.jpg (for example: filename_1024x768.jpg for an image scaled down to 1024x768). This lets you easily keep track of all the resized versions of your images.