ChadJessen take it a step at a time brother ?
Learn the basics, if you aren't familiar with HTML5 (or HTML in general) or CSS, start there first and try CodeCademy. I recently took a Linux bootcamp to up my Linux Administration skills and now I'm moving on back to HTML/CSS. I would highly recommend buying Colt Steele's "The Web Developer Bootcamp" from Udemy if you can spare 10 bucks to get it. He explains things extremely well and I believe it would be best for those who like to learn through video instructions. You'll get HTML, CSS, and JavaScript all in one go with that bootcamp and it should bring you up to speed with learning how to develop for the front-end of Flarum.
For the back-end, you'll have to pick up some PHP from Laracasts (there should be a free set of lessons that tackle the basics) and then you can move on to OOP (i.e. Classes, Inheritance, Methods, Attributes, etc).
And also if you ever need help from us, head over to the Flarum Community Discord and chat with knowledgeable developers anytime!