To reduce confusion and expedite your process, it is important to become familiar with certain permanent residency terms before you begin the application process.
Above all, stay calm. As long as you have provided solid evidence that your relationship is legitimate and you answer all questions truthfully, then you should have no problem during the interview.
To remain in the U.S. legally after your fiancé visa expires, you must submit an application for an adjustment of status.
If the alien fiancé is lawfully present in the U.S. when you meet and fall in love, it might be the best idea in terms of your life circumstance to simply get married and apply for permanent residency while remaining in the U.S.