As a first step I'd suggest you review this UKVI webpage link:-

Application for UK visa for family settlement: form VAF4A

You'll find that link contains the Family Settlement Application Form VAF4A, the Appendix 2 Financial Requirement VAF4A Form and also the Guide to Supporting documents.

The current UKVI visa fees are here

You'll see that the Settlment (Fiance) Visa application is £885
This visa has a validity of 6 months to enable you get married.

Do note that once you are married in UK you'll need to apply for FLM(M) which is Further Leave to Remain and that will cost you £601 for application.

For additional background information please take a review of this forum thread All New Guide - How to apply for a Fiance Visa