Bavarian beer, specifically. And if you can find something from one of the "approved" German brewers (Augustiner-Bräu, Paulaner, Spaten-Franziskaner, Löwenbräu, Hacker-Pschorr-Bräu and Hofbräu), all the better!
The official Oktoberfest collector's mug would be ideal, but if you can't get your hands on it, anything that resembles a beer stein, like these, will do the trick.
The soundtrack at Munich's Oktoberfest is all polka and folk music (during the day, at least), so your at-home Oktoberfest should sound the same! Spotify has every kind of music these days, so you should be in good hands.
Whether you outsource a fancy event tent or just pitch your own canopy, a good tent is crucial to an authentic Oktoberfest. Basically the entire festival in Munich takes place under a series of tents, with reservable space that's more competitive that a new Manhattan bistro. Your backyard set-up will likely be much more accommodating, but this is the intended atmosphere in which to enjoy your Bavarian beer.