The 90s were a time of teen angst, questioning authority, and expressing yourself through fashion. The grunge scene was about not caring what other people think, not playing to society's rules, and experiencing life as it comes to you.
The look is easy to get, and the best part is, it looks good on everyone.
While you don't have to be rebelling against the box society puts you in to really get this look, listening to Nirvana sure helps :)
As for clothes, its all about the layers: dresses, flannel shirts, tights and shorts, cardigans, sweaters. Dark earthy colors that bring out the rich colors in any skin tone and highlight your eyes and bone structure.
Check out the style evolution of Courtney Love for some serious inspiration!
As for the hair, you mainly want to look like you were just head banging to some seriously loud music just seconds before. Here's a great tutorial for that!