How can I become a video game designer?

1) Make stuff
Making games is the most important way to prepare for being a designer. There are a variety of free tools ranging in from complexity like GameMaker to Unreal 4 or Unity. In any case, there are tons of tutorials on the web. Start by trying to recreate very simple games like Pac-Man or Flappy Bird, then move on to more complex features.

2) Get an education
You don't need to go to a game design school. My major has nothing to do with making video games. Computer Science will always help, and is a great choice, but isn't mandatory.

3) Play a lot of games.
Practice thinking about the critically and write it down. Writing down your analysis forces you to think and communicate clearly, which is perhaps the most valuable skill designers need.

4) Network.
Look around for game studios in your area and ask if you can take a tour. Meet people and see if they're willing to give you advice. You might end up with an internship, or even simply get a chance to interview.