How to say "Girlfriend" in Chinese:

Nu3 peng2 you5

What are these numbers? 

Nu3 = girl
peng2 you5 = friend

In Chinese, the word girlfriend is formed just as it is in English - by taking the word for girl and the word for friend and putting them together.

Here are a few good combinations to commit to memory:

My girlfriend = Wo3 de4 nu3 peng2 you5
Your girlfriend = Ni3 de4 nu3 peng2 you5
His girlfriend = Ta1 de4 nu3 peng2 you5