Articles are different from static pages in a few ways, but most primary among them is that articles have the author's name attached.
First, add the author to the system
To add an author to your system, go to Content -> Add content -> Contact
Fill in the necessary fields for the author. Things like Job Title and Twitter account are optional, but will show up on this author's page if filled in. The only field that's required is Full Name.
You only need to do this step once.
Then, just type the author's name into the article's "author" field
Start typing the name, then select from the results that appear.