Not counting Treehouse of Horror episodes or dream/fantasy sequences, her first real word was "Daddy" although no one heard it. As for as the family knows, her first word in the series was "Yah" said in Season 20's Coming to Homerica, although she also had her 'first' word at the end of The Simpsons Movie in 2007 when she said "Sequel".
The unibrowed baby.
Bart, Lisa and Maggie
maggie simpson
Maggie Simpson.
There are several little girls on The Simpsons... Lisa, Maggie, Janey, Sherri, Terri, Jessica