There are several different ways to do this.
You can simply look through the list of the most "expensive" boards and choose the ones you like. The rating of boards by their capitalization is displayed on the search tab in the application or on the Buy board coins page on the site.
If you know what you are looking for, you can search for specific boards by name or url right in the search bar.
You can find the most popular boards on the topics you are interested in in the browse boards screen, which is available on the search screen in the application.
You can also view the complete board directory on the page: https://main.community/boards