You can create a post at in a few ways.
In any view within the WordPress mobile app, you can tap the box with the "+" sign in the upper right hand corner which will bring up the create new post view.
You'll want to ensure you're posting to the correct site if you have more than one site. You can change this at the top.
You can also navigate to the "Blog Posts" section by heading to My Sites > Site > Blog Posts in this view you can see your posts and tap the "+" in the upper right hand corner to create a new post on that site.