Opera is too big a subject to really discuss here, so this will be a schematic, designed to help you find the information that is already on the Internet.
In the beginning there was the Camerata Fiorentina, a late 16th century study group of musicians, poets, and scholars drawn together by Count Giovanni Bardi, whose goal as to revive ancient Greek music and drama. It's most famous participants were Bardi himself, Giulio Caccini, Vincenzo Galilei (father of the telescope wizard), and perhaps Jacopo Peri, who is usually credited with writing the first opera, La Dafne. The Camerata was long thought to be the incubator of opera, but that view has been seriously challenged since the 1950s, and another Florentine group, which met a few years later in the palace of Jacopo Corsi and included both Peri and Claudio Monteverdi, is now thought to have been more influential and forward looking. It is Monteverdi's 1607 Orfeo, which is still performed, that is really the prototype of modern opera. You can find information on all the above people on the Internet at either the Encyclopedia Britannica opera site, http://www.britannica.com/eb/article?eu=118789&tocid=27827 or by searching for them with the Google search engine http://www.google.com/advanced_search.
Britannica and Google are also good starting points for other opera information, but there are many specialized Internet opera sites. Better known composers, performers, venues, festivals are all pre-catalogued on the net at such sites as:
Opere Liriche (in Italian): http://www.promart.it/Opere/index.html
The best know Italian venues also have specific sites:
Roma, Teatro dell'Opera: http://www.operabase.com/mkhouse.cgi?house=wirmo
Milan, La Scala: http://www.teatroallascala.org/eng/homepage.htm
Venice, La Fenice: http://www.teatrolafenice.it
Naples, San Carlo: http://www.teatrosancarlo.it/uk_index.html
Ravenna Festival: http://www.ravennafestival.org/
Verona Festival: http://126.96.36.199/English/
Spoleto Festival: http://www.spoletofestival.it
(Lesser-known houses: http://www.operabase.com/en/itmap.html,
and Smaller Festivals: http://www.operabase.com/en/festivals/it/)
A very good brief opera history is at http://www.lawrence.edu/fac/koopmajo/brief.html
For an earlier item on Giuseppe Verdi, go to http://mywebpage.netscape.com/mmdtkw/VVerdi.html.
P.S.: There have always been debates on how inclusive the term opera should be, and the current ones center on whether operetta, Broadway, and or "rock operas" qualify. These debates are, of course, only pastimes for the purists. Get tickets for what you like and enjoy!