You must be registered as a User and logged in to Post. (see previous)
When logged in you have access to the Dashboard mode. From the Dashboard choose Post under New (on the top bar) or Add New under Posts (in the left menu).
You are now in the Post Editor. Here you can type in or paste text. You can include images (even videos & such) by choosing the Add Media button. You definitely should assign your post to one or more Categories and select a Featured Image.