It really depends for each person. What sparks your interest and what motivates you to write. You can always make notes on what you want to write then mix match what you like about the notes then trash the bad notes.
I get my ideas from boredom which inspires me to write. The more bored I am the more creative my imagination gets, which I think is very weird. If you can’t think of a great way to start then read some stories, it could be literature, fiction, non-fiction, etc. That could spark something for you to write and hopefully until the end.
Also if you ever find yourself in a writers block then these topics are really good to read. The answers given by the community are all interesting and worth the time to read about. You can even get inspired on how they make their games interesting if they hit a wall or they think something is not really working.
Dreaded Writers Block
Not exactly writers block?
Writers BLOCK! - What do I do?