。
The girl who went to the movies with me is my friend.
Literal
I [with-と] together movie [to-に] went girl [topic-は] I [of-の] friend is.
A relative clause 僕と一緒に映画に行った ('who went to the movies together with me') modifies 女の子 ('girl'). Note the compact Japanese structure: all modifiers precede the noun with no relative pronoun. 一緒に is the adverbial 'together.' 映画に行く is 'go to [see] a movie' using the purposive に.