How do I make a new board on 4chan?

Your question belongs in Computers & Internet>Internet>Other-Internet as it's currently in Yahoo Products>Other-Yahoo Products and not applicable to this subcategory, which is for questions about products Yahoo produced, that don't have their own specific subcategory (and thus decreases your chances of getting many useful answers).

Moving your question can be done by clicking Edit (pencil icon for mobile)>Edit category just beneath your question, then selecting the category that's appropriate for the topic.

if you type this on the search bar it might show you how to do it

you cant, thats something only the admin (site workers) can do. to make a new post, just go to the board and click on "start a new thread" at the top