Sharing Variables between Webpage and PHP

I am new to html, WP and PHP. I got my site up this weekend but can’t save user input entered on a custom form. $wpdb->insert(
creates a blank record (with some defaults). The problem is that variables populated on the web page don’t come over to the php code. Can someone give me a very explicit lesson in how to fix this. Let’s say using one variable called “FName”. Thank! …pulling my hair our here…..