what can i do to start learning software developing at an age of 15 and in 9th grade

If you have access to a computer with internet access, there are hundreds of thousands of tutorials out there to get you started. I personally think that Java is the best language to learn first because it allows you to learn about Object-Oriented Programming and get used to best practices of software engineering. I also recommend udemy because I am a visual learner and the lectures let you follow along and pretty much make it impossible to not know what to do - I feel that many tutorials/books omit important details that I then spend my time trying to figure out instead of getting to the real learning! Here's the signup link (it's free!) - https://www.udemy.com/java-tutorial/

Ollie gave really good advice. I would add, look for things that interest you. Software engineering is such a big field that you can afford to find parts that you enjoy, and you will surely practice more (which will improve your skills faster and prepare you for a career) if you are doing something you like. The initial parts are the same because the fundamentals are common across all of software engineering, but after that, try things out and find the ones that entice you to keep going. Software engineering means a huge amount of staring blankly at error messages, wondering why things aren't working, and so if you can be accomplishing something you are interested in, it can help you not get too discouraged. Most of what you learn will still be applicable to other focuses, too. I gained most of my programming skill writing games on a desktop, and that hugely helped me even though I don't write any games for my current job.

Above all, don't get discouraged. All of software engineering is about breaking problems that look overwhelmingly large down into smaller, manageable chunks, and you can apply that while learning as much as in an actual career.

Do you have access to a PC of some kind with internet access? If not get one. You really need a machine you can spend a lot of time on.

You can download some excellent software developer tools free of charge: Visual Studio Community Edition, Eclipse, and so forth. Use your favorite search engine to find them.

There are some excellent free web sites for beginners. http://codecademy.com/ is a good start. If you're looking for interesting programming exercises https://projecteuler.net/ can help you.

For a language for starting out, I suggest Python. But you should also take a look at Java, Ruby on Rails, and C#.
You've said one of your purposes is to prepare for a job that pays well. Many businesses and governments use SQL to store their data. If you're good at SQL you skills will be valuable.

This is a vast field, and a wonderful one. Your challenge today is to get started. So, go to codecademy, sign up, and follow the directions.

Look for something you want to write or a problem that you want to solve and then search for the best tools to solve it. For example, are you interested in games, do you want to build a mobile app, want your own website, etc.? Personally I find that it's much easier to learn if I am trying to solve a specific task. Then you can find the best tools, language, and some samples to get you started.

I got into this field because I liked playing video games and found that I could write my own. I started out finding games that had source code and modifying it to make it into something different.....even though I left gaming long ago for writing business applications that foundation gave me a great start and a great career for the past 25 years.