The 'X Factor' judge - who was bullied when she was at school - is the new ambassador for Secret's Mean Stinks campaign, which encourages girls to "gang up for good" to end bullying in schools.
She said: "I heard about the programme last year, and thought it was such a great opportunity. They're really trying to put an end to bullying."
The programme asks girls to pledge to have a "Drama Free School Year" by making a pinky swear, symbolised by wearing blue nail polish on their pinky fingers and Demi added: "I'll definitely be wearing [the nail polish] starting soon. It's a conversation starter: 'Hey, why is your pinky blue?' 'This is a pinky promise that I'm not going to bully people, that me and my friends are ganging up for good.' "
Demi, 20, also spoke of the tough time she had at school because she was targeted by bullies.
The singer - who spent three months in rehab for her eating disorder and self-harming at the end of 2010 - told PEOPLE: "I had a really tough time when I was in middle school. People would write 'hate petitions' [about me] and send them around to be signed. They'd have CD-bashing parties of my demos. They'd come to my house, stand across the street and yell things. It was a very emotional time for me, and all I wanted to do was get away."