Tip calculator using JavaScript

What are you most proud of, and what would you do differently next time?

I am proud of finish this at all; I love replicating websites from a picture, but the Javascript portion doesn't come as naturally to me. I think I need to do a few more JavaScript projects on my own before jumping into something like this, until I understand the fundamentals.

What challenges did you encounter, and how did you overcome them?

Learning Javascript has been slightly challenging for me, so I had to use Google a lot to finish this project. That being said, I did much more of it on my own than I anticipated.

What specific areas of your project would you like help with?

I would love to know some good resources to really learn & understand JavaScript, & general critique on anything I could have done better! If anyone cares to look this deep, I am also curious how I can improve the organization/efficiency/structure of my GitHub! Any feedback is welcome & appreciated :)