The most important first step is to decide what your wiki will be about. Have a look around Fandom to see if anyone has already started a wiki on that topic. If so, it's much better to join that wiki than to create a new one. That way you will be working with others to make a site without starting from scratch.
Once you have created your wiki, the next step is to write! Good content is what will draw people to your wiki, so building that content is the best way to start.
For more information on the creation tool and how to use it, click here.