A fiance visa must be applied for from outside the UK and the fee as of April 2015 is £956. The earliest you can apply is 3 months before you wish to travel to the UK. You can ask for the visa to be post-dated by up to 3 months (the application will ask for your travel date and the date you put will be when the visa is made valid from - this date cannot be more than 3 months ahead of the date you apply online). Gather up all your documents - financial requirement documents, accommodation...