Install Xcode on OS X 10.8.4
Old Xcode versions can be downloaded from developer.apple.com, free registration is required for this. To install (and distribute) your own applications, you'll need to register as an iOS developer. This costs $99/year but can easily be done later.

Having said that, you may not even need to develop an app for what you have in mind. In Mobile Safari you can save any web site/URL as an application icon on your home screen. When tapped on, this will open the web site as an application.