You click the "start a hub button" at the top of the page.
Choose your title and all that other stuff.
For category you want to choose
Books, Literature, and Writing > Poems and Poetry
Then choose which category your poem fits into.
Once you publish your poem and you go to the page you will see the share buttons along the left side at the top of your poem. Just click the facebook icon and it will let you share it right from there.
I recommend turning off ads for poems because I think it takes away from their beauty and makes them look messy.
Adding your own pictures or finding some creative commons pictures (pictures you are legally allowed to use and that are attributed correctly) can really boost your poem and make people more likely to share it with others.