Volfan has pretty good advice.
I may add or highlight a few things. First since you share the same class together start with topics like an up coming test or homework assignment. Ask her what she thought of it. If she found it easy just ask her how she approached it to get a good grade. From there you can ask her what she is studying or anything school related.
After a few weeks go by I would see if she would be willing to be a study buddy for an upcoming test in that class and try to meet her at the library,student lounge, or a coffee shop. If she likes you as a friend or sees something more there chances are she will agree to it.
Now that you have essentially isolated her away from the classroom setting you can do one of two things depending upon how things are going. You can continue being study buddies and classmates or possibly ask her out for something non school related. Before doing so it would help to know her interests. School sporting events, a concert, party, etc. At this point it will be up to you and her as to wether or not something is there for any future dates and possibly a relationship. You may find out you are or are not compatible once you get beyond looks.
After you have had her out for a non school related date then tell her how beautiful she is but I wouldn't start perusing anyone this way unless you are at bar looking for a one night stand.
Lastly outside of the voice possibly look into a gym membership or a home gym. It would relieve stress not to mention help reshape your body thus improving your confidence.
Good luck and remember even if you don't get anywhere with her keep trying. It may not have anything to do with you and maybe she doesn't want or have time for a relationship. Oh and always be yourself.