I got into game design at microsoft I think the biggest advice I could give is you have to practice. It seems like simple advice but if a programmer or artist is going to be good they have to practice they have to do draw or code. For a designer you need to just come up with random ideas, make new professions for your fav game, or skills, or items. League of Legend character are super good practice, write them down I can't tell you how many countless notebooks filled with random ideas I have. I have half made board games, and a
personal wiki filled with random ideas just design and write it down. Even playing a game and trying to break down how the systems of that game work. Really starting to categorize systems and ideas, as well as content types, and story. Breaking down and playing other games is an important part of learning and understanding design. In the end I think a lot of people under estimate how much practice goes into design and the more you do it the more experience you have at knowing what to avoid or how to approach a problem. It's why experience is so important when it comes to design and getting the first few design jobs is hard because everyone is looking for people with some experience so you often have to get into design from QA, community, programming or art. ~Izzy