There is no (known) built-in way to get back the Start Menu or "classic mode," but you can try:
- Start8, which will give you a start menu and allow you to bypass the Metro Start Screen altogether, but it won't altogether disable Metro, since you'll still be able to access the charms bar and the Metro task switcher.
- You can use this Classic Shell skin to get a close-enough replica of the Windows 7 Start Menu back.
- Or, if you just want to do away completely with Metro, you can use this trick to use the Windows 7 shell file in Windows 8, which will basically send you right back to the Windows 7 experience, BUT REMEMBER TO READ THE "PLEASE READ" SECTION OF THAT POST to know just what you're getting yourself into :D