I Would Like To Open Files in Different Tabs in the Same Window Using Xcode 5
Left column in Xcode is called Navigator. You can show/hide it with ⌘+0 (Zero), or in the upper right window corner, see image:
 

In the Navigator there are dfferent tabs (Project Navigator, Symbols Navigator etc.). To see list of files in Your project You need to select Project Navigator or press ⌘+1.

To open file in current tab, single click on it in Navigator. If You double click the file, it would open in new window/tab. You can change the behaviour of double click in Preferences... -> Navigation -> Double Click Navigation.