Before you get started, download XCode 6 from the Mac App Store. You can buy an iOS Developer Program for $99, but it's not required at this stage.
Start reading about Objective-C. Here's a good quick overview. A more complete primer from Apple is here. If you prefer to learn by video, Lynda has a good overview, it's about 6 hours.
Follow the tip calculator video walkthrough to get the basic tip calculator functionality. After you finish the walkthrough, follow the hints below to add a settings page.