To get to number one in the charts, you need to score over 6000 points. You can do this on easy, where your voice is given much more leeway around the notes.
If you're having difficulty doing that, then you can try putting the microphones near a fan for the duration of a song. This doesn't work as well as it does for Rock Band and Guitar Hero, but should get you a decent score.
You need to get scores over 6000 on ten different songs. I've found the following to be easier than others to get good scores on:
The Killers - Mr Brightside
The Pussycat Dolls - Beep [choose the 'Will' track]
Radiohead - Creep
Amy Winehouse - Back to Black
Blink 182 - All the Small Things
Lily Allen - The Fear