I'd personally suggest just getting used to Windows 8. About 45% of the interfacing is the same... It's really just some of the Metro menu stuff and a few of the settings that are awkward at first... But yeah... once you're used to it it's pretty nice.
That said, you can use ClassicShell as recommended, however I'm personally not really into off-brand stuff that modifies the OS... personally.
If you really want to revert to Win 7, you'd have to do it as Allan said... but you would need an OS disk and a key.