Unless you're a fan of the movies, don't buy this. If you want to sing, there are plenty of other nice titles available.
In "Sing it" you play as one of six characters from the movie, though others are played as in story mode. There is always one girl and one boy singing, and if more than two people sing in a song, the extra voices are added to the appropriate gender, causing quite a bit of confusion.
Anyone who has played any music games will know the basic gameplay, when the notes come across the screen, you sing them. What words to sing is pretty hard to tell unless you practice the song a lot, because instead of the words sliding across with the notes, the notes are like subtitles on the bottom of the screen while the notes are nearer to the top. The words highlight in blue as they are supposed to be sung, but only one measure of music is shown at a time so you will never know what notes will pop up when the next measure suddenly appears. This is extremely awkward in fast paced songs, as well as the normal ones. No bar lines are shown either, so you have to figure out on your own what note it is you are singing. Overall the system really doesn't work.
The controls are just as awkward if not more so. B is confirm, you have to point the overly sensitive barely visible cursor at the field you want to change, then go to the arrows at the sides of the screen to move your selection. It would be much better if you could just use the control pad on the remote.
Unless you are a big High School Musical fan, do not buy this game. If you want to sing, there are plenty of other nice titles out there.