The followings are all the forms you will need when applying for K1 visa:
I-129F : This is form you use to initiate the petition to bring your fiance(e) and his/her children into the United States for your marriage.
I-134 : This form is to declare that you will be able to support your fiance(e) financially so that he/she will not become a public charge.
G-325A : This is to provide biographic information of the individual.
To learn how to use these forms, read our K1 Visa Application Guide
The purpose and function of I-134 form is explained Here
