There's no "one true way" to design a game, and the approach would differ depending on your experience, the size of the team, how it's being funded (or not), whether the designer is also going to be the programmer, etc.

For a complete beginner I would recommend writing a one-paragraph summary of what the core of the game is about, and then list all the key features. Each of those features then needs to be specified in a reasonable amount of detail, with reference to how they interact with the others. Entities in the game (such as characters, enemies, weapons, items, etc) also need listing, so that there is an idea of how many assets need creating. From there, you can start building, and design things in more depth as you need to.

I don't recommend working on a traditional 50-page design document - nobody is going to read it except you, and you'll find yourself needing to change it as you go along anyway.