Recommendations for Getting Started with Plugin Creation

I don’t have a coding background or anything and I am just wondering what I should read or watch to get started in learning how to Code. I do understand basic HTML and CSS but I know I will need to learn some PHP.

The plugin I first want to create is basically an Addon for Gravity Forms that would allow people who fill out a form to register for GoToWebinar using the GTW API. I have the developers license to Gravity Forms and I know they said I can reverse engineer one of the existing Addons to figure it out, but I am still not exactly sure where to start.

I’m all for reading books, or watching videos to learn. I really appreciate all the help I have gotten so far on these forums.