The Oracle at Delphi was probably the most important oracle of the Classical Greek world. The Oracle, was the priestess at the Temple of Apollo at Delphi and was widely credited for her prophecies inspired by the god Apollo. I'm not sure how the priestess was selected that might be something you may want to look up.
Numerous treasuries were built in honour of the advice of the Oracle and were built by the various Greek city-states to commemorate victories and to thank the Oracle for her advice, which was thought to have contributed to those victories. The reson they're called treasuries is that they held the offerings made to Apollo; they were normally a share of the spoils of the battle or war. The most impressive is the Athenian treasury, built to commemorate the Athenians' victory at the Battle of Marathon.
If you need to know about rituals and how and why the Oracle was consulted then good sources of info might be the ancient writers Herodotus and Pausanius . Herodotus wrote an extensive history of the many Greek states and covers both wars with Persia, when the Oracle would certainly have been consulted by numerous cities. Pausianius wrote a, kind of, tourists guide to Ancient Greece during the much later Roman period. There would certainly be a section on Delphi, I had to use Pausanius at uni and I would be suprised if he didn't tell you about rituals and the history of the Delphic Oracle.
Here's were you can find Pausanius's work online http://www.theoi.com/Text/Pausanias10A.html
(Had a look and section on Delphi starts at - Book 10.5.5 - it continues for a while so there should be plenty of information on the oracle)
Plutarch may also help you, he served as a hig priest on Delphi at some stage and wrote about in his 'moralia' heres an online version of it http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Plutarch/Moralia/De_defectu_oraculorum*.html
These may seem a bit daunting to read, the best way to aproach them would be to skim read them looking for what you feel is relevent. They should all have all the sorts of answers you are looking for.
The island of Delos was a major sanctuary and one of the holiest places for Athens and other Ionian Greeks. I can't remember too much about it, except that the Athenian led Delian league held meetings there and it housed the treasury of the league when the league was formed following the Second Persian War. But if you look look up the island of Delos you may find important information relating to cult worship and oracles.
Hope you find that all useful
P.S. The end of the Plutarch text appears to give a discussion on the fumes the oracle used in order to communicate with Apollo, may be somthing there!
BA in Ancient History