。
I have to go now.
Literal
I [topic-は] already go-must-[なくちゃ].
~なくちゃ is a casual contraction of ~なくてはいけない ('must do'). 行かなくちゃ = 行かなくてはいけない. もう ('already, now') adds urgency. Extremely common in everyday spoken Japanese — one of the first casual contractions learners encounter.