Stewie works at Enterprise Rent-A-Car. This is the same place Mark Wahlberg's character works at in Ted. Stewie is voiced by Seth Macfarlane who wrote and directed Ted.
The way characters can (or cannot) understand what Stewie is saying has gone through two major iterations through the series run. Earlier on, it was a constantly changing dynamic where the same characters could sometimes understand him and sometimes not, and it was a prominent part of several episodes. Around the halfway point of its 23 seasons thus far, though, the show made it clear everyone except Lois and Peter can hear him when he talks. It also made this trait a non-factor in storylines.