Ok, there are several ways you can do this. Either,
A) Use it as the first or last line in a story.
B) Your character thinks this (woah baby) when he or she sees something they cant belive.
C) Talk about a baby. When they see the baby, make them say “Woah! Baby!” Or
D) Make it something no one would even think to expect. If your going one way with the story completely take it in the different direction in the end. Make your read think “WOAH BABY!!”