When you suscribe, by default, you have the right to comment the articles of other people. (subscriber role)
If you want to write your own articles, send the webmaster an email and he will upgrade your account to the contributer role.
Then, when you have this access rights you, log in and use the left part of the interface to select: “Posts/Add new” command.
Then insert a title below “add New post” and a text content below “paragraph”.
You can also insert pictures by using the icon on the right of “Upload/insert”
Finish by clicking on the right part on the button “submit for review”
For more details: