Hi priscillah
Are you talking about making a customised version of the official Moodle Mobile app, or making a completely separate iOS app to communicate with Moodle?
If the first one, they have a guide for setting up your development environment and some developer documentation.
If the second one, this is primarily a forum for Moodle development discussion, so you'll need to look elsewhere for general help with iOS development, but the Moodle side of things would be achieved through the Web Services. I would recommend you look at the official app first though, if you haven't.