You need to contact your local registry office at least 16 days before you would like to get married. They need to display the request for 15 days so people can object for some reason.
The will help you with forms etc.
https://www.gov.uk/marriages-civil-partnerships/overview
Minimum age 16
Both partners must be acting by their own consent.
Both partners to be married must be of sufficiently sound mind to understand the nature of the marriage contract.
You must not be related to each other in a way that is forbidden by Law
Not already married.