The best way of initiating conversation with a girl rather depends on where you are and what the situation is. If you are both studying together, it is easier because you can make a statement about th study material and ask if she agrees, or flatter her by asking if she has understood it rather better than you.
It's a good way to get into conversation if you start by saying you found something confusing, or difficult and you may find she has felt the same. Then you have a mutual experience to start off a friendship. It might even be that you say with a smile, 'I found that a bit boring/really interesting, did you?' as you leave a class.
If you are not students, and you have not met th girl before you need tofind an introduction. This is a different question.