Developing with iOS7 requires that you upgrade to Xcode 5, which you can find free in the Mac App Store.
You'll still be able to keep your deployment target below 7.0 with Xcode 5.
If you don't have a dev iPhone with iOS 6.x you can go into Xcode > Preferences > Downloads to install the iOS6 simulator, command line tools, and any additional documentation.
Update 1
If you update your phone to iOS 7.1, you will also be required to update your version of Xcode to 5.1