Updated: If you don't mind spending 240 Microsoft Points, Fatboy Slim's "Weapon of Choice" DLC is probably the easiest song to get this on.
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Whatever song you feel most comfortable with, that's the one you should go with. For me, it was Poker Face (on Easy). You could use Break It Down to practice, but I just kept performing it over and over.
You will need to get a Flawless on every move, including the finish. Unclear as to whether or not a Flawless Freestyle is necessary, but I'd just wave your arms in the air while the camera is going off just to be safe.
Have patience. Even on Easy, this one can be daunting.
EDIT: tsunamishadow recommends the Warm Up Challenge as opposed to Poker Face. So if you can't nail one, try the other.