For general requirements of British Citizenship look here:-
http://www.ukba.homeoffice.gov.uk/britishcitizenship
For detailed requirements as spouse or partner look here:-
http://www.ukba.homeoffice.gov.uk/br...rtnerofcitizen
The Application Form and the Application Guide can be dowmloaded here:-
http://www.ukba.homeoffice.gov.uk/br...naturalisation
Generally these are the requirements:-
you are aged 18 or over; and
you are of sound mind; and
you can communicate in English, Welsh or Scottish Gaelic to an acceptable degree; and
you have sufficient knowledge of life in the United Kingdom; and
you are of good character; and
you are the husband, wife or civil partner of a British citizen; and
you meet the residential requirements; or
your husband, wife or civil partner is in Crown or designated service outside the United Kingdom.
General residential requirements
have been resident in the United Kingdom for at least three years (this is known as the residential qualifying period); and
have been present in the United Kingdom three years before the date of your application; and
have not spent more than 270 days outside the United Kingdom during the three-year period; and
have not spend more than 90 days outside the United Kingdom in the last 12 months of the three-year period; and
have not been in breach of the immigration rules at any stage during the three-year period
BTW, you can count residence from the first time you arrive to UK, even if it was with Fiance(e) visa.
Unless, it changes next April