My friend told this girl who likes me that I liked her 2... The thing is, she doesn't believe him...... What should I do
If it is true that you like the girl, the best thing to do is to approach her and tell her personal that what your friend said is true. The reason as to why she did not believe your friend could be because it was not from your mouth.
Try approaching her yourself instead of relaying messages through your friend.
Grow up and tell her yourself.
ask her out