Emo punk hairstyles are generally understood to begin with black straightened hair. A long fringe or bangs are usually brushed to one side or the other. The back may be short or long and is typically weighted down with gel or hair spray. If you want the emo look you will have to suffer through growing out period problems.
Early emo punk hair fashion looked like Romulans and Vulcans in the Star Trek movies. As the hair styles evolved, they remained similar on the front and sides of the head, but the back became longer, spiked and more asymmetrical. Black hair was the norm in early emo styles. Today you see black, platinum and other colors, but these can appear clownish if you go too far in that direction.
The hair in front and sides should be long enough to cover the face. It will need to be thinned and shaped once the grow out is sufficient to not look ragged. The style will need regular cuts and care in order to avoid the appearance of hair that just happened. You should also plan on investing in a straightener.
Hair care is important too. Having emo hair doesn't mean dirty hair. You will need to have hair that is shiny clean to start with. The hair spray or gel on your hair to style it will mean regular washing. You will need to use a moisturizer that suits your hair type to avoid drying and the straw appearance that doesn't look emo, only scarecrow.
The nice thing about emo punk hairstyles is that you can work with the genre, but still a style that is uniquely your own. You can work with length, height and spiking to perfect the look you want. You just need to use your imagination loose.