This newsgroup is perhaps the BEST source of information you'll find. It's full of wonderfully helpful people who are going
through the very same process.
The K-1 Visa FAQ page, which contains links to many immigration sites including Fiancé(e) and Spouse Visas.
The Official United States Citizenship & Immigration Services page.
(this used to be the Immigration
& Naturalization Service)
Questions and answers on dual US/other citizenship