These are the default steps that you need to do in order to add a blog post:
1. Go to Posts > Add New
2. Enter a title and some content.
3. Select a post category.
4. Add some relevant tags.
5. Choose a post format from the right. There are a few types of custom formats that can be used, and each one of these can be setup in the Post Media / Content meta panel.
6. Setup the content of the post format(either a gallery, a link, an image, a quote, an mp3 or a video). All the fields are properly documented so you should have no issue on setting up your content.
7. Insert all of your remaining content in the content area. You can have images, paragraphs, etc..
8. Write a few words excerpt(it is good for search results and SEO to have an excerpt, no matter what kind of content do you have in your post).
9. Hit “Publish” and you’re all done.