I am not signing up for an account so I don't know exactly how the API works, but if I were setting up an iPhone App to work with this, I would use the rails plugin. Rails gives you JSON communication for free (IE no extra work required) http://guides.rubyonrails.org/layouts_and_rendering.html see section 2.2.9. Then use on IOS app use something like: http://code.google.com/p/json-framework/ to interact with that.
Before you can send AWesome emails with AWeber, you'll need someone to send it to. The easiest way to begin building your list is to import contacts that have opted-in to your email list, or gave you permission to send them emails. These could be subscribers you've emailed from another provider, or a list of contacts that you've built through your business.