The resolution you should use depends on how you plan to use the images you are scanning. If you plan to post your images on the web a resolution of 72 dpi should suffice because this is the common resolution for monitors. If you plan to use your image in a print document such as a newsletter, you should scan your image at 200-300 dpi or higher, but note that you will get a much larger file at these high resolutions.
A good practice is to scan your pictures at the highest resolution you can, then use an image editing application to save a copy at a lower resolution. That way you will have the flexibility of being able to use the scanned image in many different types of projects, from websites and Powerpoint presentations to newsletters, posters and other print media.
Another thing to consider when you scan your images is the color depth. Make sure you have this setting entered correctly in your scanning software. If you are scanning a black and white picture, for example, it makes no sense to scan it using the same settings as for a color photograph. You will end up with an unnecessarily big file if you do not properly set your color setting.