When you say "Classic" I'm guessing you mean OS 9, in which case SheepShaver is the way to go. Get the Classic-on-intel (COI) package from your bittorrent tracker of choice, then, after following the provided setup instructions, replace the bundled version of Sheepshaver with the latest build (from
here). It should work fine after that.
If you are referring to pre-OS 9 apps, then you might want to try Basilisk II (for 68k and OS 7/8 apps), or even mini vMac (for Mac Plus era software)