Well, let’s see. You have approx. 2.1-3 years before you turn 16. Plenty of time to become a freelancer if you stick with it. I built a simple program for my church to open and switch between songs quickly when I was 15 (I started programming about 6 months prior). I did my first “paid” job moving a friend’s site to another host when I was 16 for $50. I also built some free plugins for people during that time. Unfortunately, I haven’t had the time to turn it into a business, but I at least have an idea of what’s needed.
If you want to freelance front-end work and build web pages, become very good at HTML, CSS, design, and animations. Learn how to properly code with and without a framework (i.e. Bootstrap). Learn UX, and how to take a client’s ideas (whether good or bad) and put them on a webpage that will properly reach out to their potential customers. Just making simple HTML pages isn’t going to cut it, you need to learn how to market ideas using a website as a medium.
If you want to build web applications or other types of software, you will need to progress to a real programming language (HTML and CSS are markup, not real programming languages). JavaScript is good because it is so widely used on the web, but you will do fine if you choose to learn another great language like C# or Python. If you choose this route, you will need to learn the basics of the front-end (how to build a webpage with HTML, CSS, JS), but not so much the design and UX. You will also need to learn the back-end (how to run servers and manage databases). You will have to be able to create solutions. Take a problem and craft a complete solution to solve it. For example, a library needs a way to understand its members better. You will make an app that enables the library board to properly track all the books/movies being brought out and give data analysis about them. The board can track and manage everything on a web app. Library members will be able to get notifications and order books through a mobile app. You will have to manage data storage, servers, provide data analysis dashboards, and build both front apps (a web application for the board and a mobile application for each library member). You will probably outsource the design and UX to a professional who specializes in that like above.
Just a broad overview of things I’ve picked up from my own experience and talking with my friends (one which is a president of a software firm). I didn’t really want to focus on the front-end, so I’ve been stuck in the second category which presents a ton of stuff to learn. If you decide to focus on building web pages, you can probably be able to make a part-time job out of it in a year. If you want to learn to do more, you will then have to learn so much more, and probably won’t be able to do much for several years.