Many/most image editing software tools provide the ability to resize your image. The most common resizing is to scale down the image. When resizing an image this, you will want to maintain as much clarity as possible. This usually means that you do not want the image scaled merely by having pixels thrown out or duplicated identically. Rather, you probably want the software to do some level of interpolation to smooth the image.
One of the packages we will be using the semester is called GIMP (GNU Image Manipulation Program) available for free for both Windows and OS X (Win) (OS X). When resizing an image it allows you to select from nearest neighbor (called none in GIMP - it basically just throws away pixels), bilinear (will basically smooth based on pixel at a location, and the four pixels immediately adjacent to a pixel in the original), or bicubic (will basically smooth based on the pixel at the location and the other fifteen pixels in a 4x4 grid around that pixel in the original). For many images you might not be able to tell the difference. For others, it might be quite noticeable. The quality can be effected by things such as the appearance of fine lines or diagonal lines as well as the degree of scaling.
To resize the image, first open it via the File menu. Next, in the window containing the image, go to the Image menu and select Scale Image. You will then be presented with a dialog box. Under the Image Size category you can specify the width and height of the new image (typically in pixels, but you can also specify to resize by a percentage or to a specific size measured in other units such as inches or millimeters), as well as alter the native resolution of the image if you wanted to for some reason. You can select from the three previously mentioned types of interpolation under Quality category.
When resizing an image, you will typically want to maintain the same aspect ratio (the ratio of the width to the height of the image). If you do not, the image will be stretched to fit the new width and height specified (if you want to change the aspect ratio without stretching, you need to change the canvas size and crop the image). By default, there should be an icon next to the width and height that appears as three chain links, linked together. This means that as you adjust one dimension, the other will be automatically updated to maintain the original aspect ration (ie: the width and height are linked). If you click on this icon it will change to a broken chain link, and the two dimensions will no longer be linked - you can change both to be whatever you choose.
Different cameras use different aspect ratios when creating an image. This is usually based on the aspect ratio of the sensor, though some cameras allow you to capture to other aspect ratios (in which case they simply pre-crop the photo when saving it). The most common ratio current seems to be 4:3 (the same as that of monitors). If you want your image to be viewable on a common screen size within a browser window, I suggest a height of either 600 or 450. For an image with a 4:3 aspect ration, that would mean either 600x450 or 800x600 for landscape orientation (wider than tall) and would suggest 450x244 or 600x450 for portrait orientation (taller than wide).
IMPORTANT NOTE: When editing an image, save the results to a NEW FILENAME. I would suggest doing this right away, and then beginning your editing. This will prevent accidentally saving over the original later. You will typically want to have the full-sized original around for other uses. If you are going to (for example) print the image, you want as many pixels of original information as possible. Also, over the long-term, display sizes will increase in resolution...