How to say "Girlfriend" in Chinese:
Nu3 peng2 you5
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:
Your girlfriend = Ni3 de4 nu3 peng2 you5
His girlfriend = Ta1 de4 nu3 peng2 you5