The guidelines of becoming a British Citizen are pretty straight forward and available online:

There are different ways to become a British citizen. The most common is called ‘naturalisation’. You can apply for British citizenship by naturalisation if: you’re 18 or over

you’re of sound mind

you’re of good character

you’ll continue to live in the UK

you have met the knowledge of English and life in the UK requirements You must usually also have: lived in the UK for at least the 5 years before the date of your application

spent no more than 450 days outside the UK during those 5 years

spent no more than 90 days outside the UK in the last 12 months

been granted indefinite leave to stay in the UK, ie there’s no specific date that you have to leave

had indefinite leave to stay in the UK for the last 12 months not broken any immigration laws while in the UK



The website goes on to explain that "There are different requirements if your spouse or civil partner is a British citizen." Those requirements can be found at https://www.gov.uk/becoming-a-british-citizen/if-your-spouse-is-a-british-citizen.

So to answer your specific questions:

What can I prepare in advance so I can spread the cost over before applying.

There are 3 aspects of getting citizenship that cost money. The first is applying for citizenship (currently £874). The second is taking the Life in the UK test (currently £50). This can be taken as part of the settlement process. The third is settlement. There are a number of different routes for settlement each with their own costs. Details are at https://contact-ukba.homeoffice.gov.uk/visas-immigration/settlement/applicationtypes/. One route is through SET(O) (currently £1839). Settlement does not expire so you can space out the settlement and citizenship fees.

Are there any tests to get a citizenship.

Yes, the Life in the UK test mentioned above

Do I need to earn over a certain amount yearly to qualify

Not for citizenship per se, but there is often an income requirement to get indefinite leave to remain/settlement

Does my nationality or ethnic origin involve different processes

Again, in general not for citizenship, but can for settlement.