Origin: The use of "hello" as a greeting on the telephone is credited to Tomas Edison (Alexander Graham Bell wanted the more naval "Ahoy!"). The word itself comes from the Old High German "halâ", which was, apparently, used when hailing ferrymen.
Hello appeared in litterature as early as 1833, e.g. The Sketches and Eccentricities of Col. David Crockett, of West Tennessee.
Sources: The BBC television program Qi and Wikipedia