One of the easiest and most reliable ways of finding the value of old stamps is to take them to a professional appraiser or other expert. You can also find the value by comparing the stamp to similar stamps for sale on sites such as eBay.com.
The hardest part of determining the value of old stamps is that very small changes can affect the price. For example, a tiny tear can drastically reduce the value of the stamp. There can also be small printing details that can affect the value, and these may not be obvious to the untrained eye. Researching the history of the stamp and using a resource such as the Scott Postage Stamp Catalog can help. However, when using printed resources, be aware that prices can fluctuate wildly, and outdated catalogs may no longer be accurate.
For larger collections or stamps that are potentially valuable, it may be worth it to visit a trade or collector's show. These shows often have presentations about how to value stamps. They also have stamps for sale, which you can compare to your own stamp. Many experienced stamp collectors are also eager to share their hobby with newcomers and can offer personalized expert advice for valuing your stamps.