I'm a girl and i'd LOVE to be able to sing like that. kinda like the girl in flyleaf?
and id also love to learn how to play hardcore guitar.
any advice?
Get a voice teacher who specializes in the type of music you're interested in. A voice teacher should be able to work with you to get the sound you want in a healthy way. Healthy is the keyword. I'm not sure what you mean by "hardcore" but anything loud and harsh has to be done correctly or you can cause serious damage to your voice. Studying with a qualified teacher is always the best bet.
Good luck!