Xcode is the graphical interface you'll use to write apps. Included with it is also everything you need to write code for iOS 8 with Apple's new Swift programming language. It's also only available for Mac, so if you plan on making iOS apps, you'll need to be running OS X.
Apple's IDE (Integrated Development Environment) for both Mac and iOS apps is Xcode. It's free and you can download it from from Apple's site. Xcode is the graphical interface you'll use to write apps. Included with it is also everything you need to write code for iOS 8 with Apple's new Swift programming language. It's also only available for Mac, so if you plan on making iOS apps, you'll need to be running OS X.
While Apple's pushing Swift pretty hard these days, you can program iOS in any number of languages, including Objective-C.
It's no secret that Apple's App Store Review Guidelines are incredibly specific. Apple has a very specific view of what apps they'll allow in the store, so it's useful to get to know their rules before you even try to make your app. If you don't, you might spend your time making something that Apple won't allow into the App Store. When you finish your app, you'll submit it to the App Store and it'll be reviewed based on a content, design (more on that in the next section), and technical details. So, head to the Review Guidelines page and get to reading. Apple also has a list of the common reasons apps get rejected here. Typically, it's because of crashes, broken links, advertisements, or incomplete information. Apple's also known to block apps that include any type of adult or political content.
It's good to keep in mind that Apple tends to be very conservative with their app review process. Chances are, if you're making something even remotely risque, it will be rejected, so be mindful of that before you start making your app.
Beyond Apple's review guidelines, they also have a set of design and interface guidelines. Apple wants all the apps in their store to have some type of consistency, and while this doesn't have to mean good design, it does mean apps use the same basic UI elements.
To get a grasp on this, check out Apple's Human Interface Guidelines page. Here, you'll find the basics of what they're looking for in both apps and icon design. They also have a set of Do's and Don'ts that distill that massive guide down a little bit so it's easier to get started.
Thankfully, Apple doesn't leave you totally in the dark on how to make a well designed app. Here are a few resources to get help you design something worthwhile:
Designing Great Apps: Apple collected together some of the best talks from WWDC about design to help you get started with designing an interface. Designing a User Interface: Apple put together a guide for designing interfaces in iOS 8 using the built-in tools of Xcode.