The link you provided is giving a 404 and I am not entirely sure what you’re referring to. Therefore, would it be possible for you provide a new link and screenshot of the issue, please?
Also, the correct way to add a post will depend on what you’re trying to do. If you want to create a post then you will need to create one using WordPress Admin Dashboard > Posts > Add New Post.
If you want to display a post or multiple posts, you will want to use one the Posts modules.
The question is a design question. I know how to add a post, how to create a page, how to play with the Posts module to show a single post or multiple posts. But I don’t know how to set a template to my posts: I have all this blank space surrounding them