Start with what you know. What do you know? Do you know anything about any programming languages?

I would start by learning how to use the 'search' function and Google in general..

What app are you trying to develop ?

Join or start a computer club at your high school. There is nothing like peer pressure to motivate you.

Join some programming forums and read the posts and ask lotsa questions.

Learn the basics of software and programming regardless of language. There are certain basic ideas that are reflected in most programming languages such as variables and loops.

Find and take an aptitude test to see if it is something that suits your personality.

Join the chess club and learn to play a musical instrument. Both activities condition your brain for logical thinking.

Above all, have fun doing it.

Udemy.com has some good video courses on programming, and a few are free or cheap. As a professor of game programming, the best advice I can offer is: practice, practice, practice.

Reading books, watching videos, etc. can give you a good overview of the basics, but the only way to learn programming is to do programming.

Come up with an idea. Keep it short and easily doable. Then build it. Use the web and programming forums to help you when you get stuck. If you are at a loss for simple ideas, clone other people's ideas.

As to what language: start with a scripting language, like Python, Javascript, or Lua. They are flexible and forgiving.

Once you start to master the basics (loops, if statements, etc.) move on to a more professional language such as C/C++, C#, or Java. They are all pretty similar and have their good and bad points. For apps, C++ and Java are preferred.

However, if you just want to make apps and not worry too much about programming, there are sites and tools which mask the programming and allow you to design and build an app without getting too deeply into the nuts and bolts.