If you have a paid account, you can access the Xcode 5 beta (it's not available publicly yet because it's not out of beta yet,) by going to:
developer.apple.com
Click on the "Developer Center" link for either iOS or OS X, whichever one you have a paid account with.
Make sure you're logged in.
Then at the top of the iOS dev center you should see a link for "iOS 7 SDK beta" or, if you're in the OS X dev center, "OS X Mavericks". Click on that.
After clicking on that, scroll down to the Downloads section of the page, find the latest version of the Xcode 5 (currently it's Developer Preview 5) and click on the "Download" button next to it.
I'd like to emphasis that Xcode 5 is in beta, and if you're not working on an application exclusively for iOS 7 or OS X Maverick, neither of which are yet released nor have release plans any more solid than "Fall 2013", you should be using Xcode 4, which you can download from the Mac App Store for free.