You know that you can find Justin Bieber posters at any store.
Anyways, to add more details and make it look a bit crazy, as fans are a bit crazy,
1) Take a Justin poster and with a black marker and make an arrow from him that says, I Love You!
2) Make a few hearts around him and make a little cupid.
3) Colour it with bright colours.
4) Your friend can decorate the rest according to her choice.