Welcome to the forum
.
There are two forms that you need to fill in:
- the online application form on the Visa4UK website
AND
- VAF4a Appendix 2 form, which you need to print and fill out by hand.
The online application is just a generic form to give your personal details, while the Appendix 2 form actually asks all the questions about how you meet the requirements for a fiance/spousal visa. Without both forms, your application is not complete and cannot be processed.
There is a link to the paper forms on the 'Join Family Living in the UK' pages on the UKVI website, in the 'How to Apply' section.
The direct link to them is here:
https://www.gov.uk/government/publications/application-for-uk-visa-for-family-settlement-form-vaf4aThe PDF of VAF4a is the paper version of the online application. It can be used for reference to prepare, but it is not accepted in the US, so you need to complete the online application instead. The date you submit the online application and pay for the visa is your official application date, so don't do this until you are completely ready to mail all your documents for processing.
The VAF4a Appendix 2 is what you need to print and fill out by hand
There is also a suggested supporting documents list on that page too.