It is probably a 40 Lire stamp being from Italy.
Identify the exact stamp using a catalog. This may involve learning about perforations, watermarks and colors as well as condition. The catalog will provide a value. The value is what a collector could expect to pay for a stamp in fine/very fine condition. If selling, most cases you would be lucky to get 75% of the catalog, unless it is very valuable, then an auction would be worth looking into. The most common American catalog for identification is Scott's. Others are Stanley Gibbons, Minkus and even the US Postal Service Catalog of stamps.