I've found how to do this on the desktop, but haven't noticed how to achieve this on any of my iOS devices. I'd also be happy to know if this actually isn't possible.
Hmmm, it's not possible. The built-in Notes app is quite straightforward. You should look for another app which will allow you do to this. There are tons of Notes app (free or not) on the AppStore.
edit: As Dori said, Evernote is really good and allows you to sort like this.