That's good news.
Your immigration journey is almost complete and now you are on the road to possible dual citizenship.
Well done.
For general requirements of British Citizenship look here
Detailed requirements for naturalisation if you are married to or the civil partner of a British citizen can be found here
Here is the Application Form AN
Here is the Application Guide and Booklet AN
The key requirements are here
Generally these are:-
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
Your time in UK as the holder of a Fiancee Visa CAN be counted towards Citizenship.
Look here for the UKBA Fees and you can see the application fee of £851 does NOT include the ceremony charges which amount to an additional £80
You will receive details of the Citizenship Ceremony with your Home Office approval letter. You will need to book a ceremony locally. Usually it'e fairly quick and shouldn't take longer than a couple of weeks, although it does depend just how busy they are at that time.
Hope this helps