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.