who wrote the song?
Justin Bieber, Christina Milian, Tricky Stewart and The-Dream.
Baby (Justin Bieber song)
The track was written by Bieber with Christina Milian, Tricky Stewart (who worked with Bieber on a previous single "One Time"), 
where was justin bieber's baby music video filmed The accompanying music video takes place in a mall/bowling-alley setting.
wiki:24280290
Baby (Justin Bieber song) [SEP] Introduction
"Baby" is a song by Canadian recording artist Justin Bieber, alongside rapper Ludacris. It was released as the lead single on Bieber's debut album, "My World 2.0". The track was written by Bieber with Christina Milian, Tricky Stewart (who worked with Bieber on a previous single "One Time"), and R&B singer The-Dream, and produced by the latter two. It was available for digital download on January 18, 2010. The song received airplay directly after release, officially impacting mainstream and rhythmic radio on January 26, 2010. The song is uptempo R&B, blending together dance-pop and hip-hop elements, while using influences of doo-wop music.