This achievement is for scoring Superstar while in co-op mode on a song. You can select co-op mode when you pick a song and are choosing which dancer you want to be, there's a "Press X to turn on co-op mode".
The big difference in In co-op mode instead of separate scores for each player, there's a single score shared for all.
In single player Superstar comes after 5-stars (ie it's essentially the 6th star), and happens when you hit 11,000 points on a song (I suppose then in co-op with 2 people it's 22,000? Not sure). You'll know you hit Superstar as it'll flash "Superstar" on the screen where your dancer is, and instead of 5 gold stars you'll have 5 blue stars.
Note that for co-op mode to be available:
A) the song has to have at least two dancers (duet, trio, etc)
B) there has to be two physical players
As such you need a buddy to do this. You might be able to swing multiple players if you have multiple phones and are playing via mobile device, though even if you got multiple players that way, I'm not sure how you'd pull off Superstar on the song (as you'd have to dance simultaneously two parts with two devices).
As for good songs to do this on, that's largely personal preference. I did this with my 8-year old daughter on 7 Rings by Ariana Grande, but just pick a song you & your friend can do well on and give 'er. :)