There are no right or wrong ways to start writing a story. Some writers will plan it all out ahead, even using a storyboard as they do for drawing cartoons. Not everyone finds this to be the way that works for them. Not everyone can get into planning and then get out of it again and actually start writing. I’ve known someone who stalled with planning a project and then never actually left the launchpad.
For those who don’t want to start with a lot of planning start with a few characters, one main problem to deal with, one main setting and a limited time span. See how it goes. Do you feel other characters evolving into the storyline? Do you get an idea for more places and more details to come along and muck up things with solving the problem? Then it seems you’ve got the tail of a story. Hang onto that tiger and keep writing.