There are a lot of good writing apps out there that each have their pros and cons. You could use the Notes app that comes free with iOS. You may also want to consider using a cloud service so that you can access your poems on your computer as well as your iPad. I like using Dropbox and Google Drive to keep files in sync between my computer and my iPad. Both are free.
My favorite writing app is Notability. Its not free, but its only $2.99... less than a latte
Hope this helps!