The film's her first and it's a big step for the pop star.
But she's pretty confident: "It wasn't easy for me. The whole transition was a little scary at first, but I'm so happy I did it."
Brits plays an American teenager who goes on a road trip with her mates and boyfriend to find her mum.
The film deals with some difficult issues that teens are faced with, like sex.
Even though these problems are treated delicately on screen, she admitted she wasn't sure whether parents would be too happy for younger kids to see it.
"I think that it's up to the parent to decide," said Britney, but she added: "My sister's 10 years old and she went to go see the movie and she loved it."
The film also includes the track I'm Not a Girl, Not Yet a Woman. It hits UK screens at the end of March.