For the best view, prop your mirror against a wall or on the back of a door. Make sure you put it somewhere with ample space to move around and dance.
Justin names Michael Jackson among his influences, and this is especially apparent in the video for his first single as a solo artist, "Like I Love You." In it, he dons a hat and gloves, and emulates Michael's smooth movements. To become familiar with Justin's work, this video is the best place to start.
Once you've gotten Justin's basic attitude and style, move on to more advanced videos. "Rock Your Body," for example, features more complex dance movements.
After you've watched all of his videos, check out Justin's concert DVDs. These are more dance-intensive, as music videos often feature a bit of storyline. In concert, Justin fills up the time by dancing, often using the entire stage. You'll also notice he goes beyond choreographed moves--there's improvisation involved. Try this, too, when you're comfortable.
To accompany your self-taught moves, try learning from professionals by taking a hip-hop class. You can build on these basics to pick up harder moves.