The best things to know when writing games are novel-writing and screenwriting. Both will help you in very different ways:
Screenwriting: Develop dialogue skills and expressing your ideas with no sound.
Novel-writing: Deep character development, crafting your own worlds, extending a small story into something epic and creating profiles for character classes.
In general, both are a good idea to learn. However, most writers I know in the industry started as screenwriters (at least in story-driven games like Uncharted. The Elder Scrolls, Fallout, Doom (2016) and Gears of War used novel-writers most of the time)
Overall, it depends on what kind of writer you want to establish yourself as. Not to say you can't do both, but focusing on one for a while before switching to another might be a good idea. Nevertheless, a diverse portfolio of scripts, books, novellas, etc. could help show your skill.
www.writersdigest.com
www.nofilmschool.com
www.gointothestory.com
These sites will most definitely help you learn the basics and expand your horizon. Good luck!