Unless you are extremely fortunate, at least a few of your old photographs will need either restoration or conservation (or both). There is some confusion about these terms: they are not the same thing. But both can help you get the image you want.
Restoring a photograph is the process of re-creating the original appearance of it insofar as it is possible. With photographs these days, this is often done digitally. Software is used to virtually stitch together torn sections; eliminate dark spots or stains; brighten faded images and more. These days, you don’t have to be a professional to try some of these techniques in photo editing software. But you do need to have the right software and know how to use it. The best results still come from trained professionals, though.
Conserving a photo means treating the actual photo to prevent further deterioration. Photographic conservators are specially-trained individuals who understand the chemistry and construction of different kinds of photos. Sometimes they can reverse physical damage, such as to tarnished daguerreotypes or stains caused by acid or mold. In the case of catastrophic damage by fire or flood, they may be able to help recover water-logged or smoke-exposed photos.
Learn more about fixing damaged photos yourself or with expert help in my book Preserving Your Family Photos.