I typically build WordPress websites for clients. I've built tons of custom themes and modified WordPress to do LOTS of different things. You know what my best asset has been while building custom themes? The WordPress Codex. I also build in a lot of functionality with jQuery plugins - like Flexslider, OwlCarousel, and ImagesLoaded. And you know how I get those to work correctly? I look at their documentation.
Documentation is key when it comes to making things happen. Eventually you're going to manipulate certain plugins or other people's code for your own particular circumstance. And when this happens you'll need to dive into their documentation (hopefully they have proper docs written!).
So unless you're writing your own plugins, frameworks, and other fun features - then you're probably using something that's already been coded. So play it smart and look for code with proper documentation. You'll get a better understanding of how that particular script works and that right there should allow you to manipulate it.