The Scratch Wiki already has a lot of articles. Make sure you search to check that the article does not already exist in the Scratch Wiki. If an article on the topic you want to create is there, but you think people are likely to look for it under some different name or spelling, learn how to add a redirect with that name; adding needed redirects is a good way to help the Scratch Wiki too. If it isn't a redirect, make sure to read the new page policies.
Consider adding your information to existing articles that might include information about the subject of the article you propose. This is the best thing to do if the subject of the proposed article has only limited depth.
If no suitable articles already exist and the subject has enough depth for an article of its own, then you need to start a new article. If you're not an editor, you can request an article to be made here.