A lot of black, the cheap black plastic choker is pretty much a universal sign, ripped jeans of any colorr really, stripes, the way you part your hair, and the biggest part: attitude. You gotta have a "FUCK THE SYSTEM" mindset and hang around as many places where there's alternative music as possible and meet chill people.
Thrift stores! You can find gems in thrift stores and use the clothes to make your own!
Any retail store, just look for black everything, and you can alter them, make your plain black jeans look cool by ripping them and adding DIY patches of your fave bands. Save any money for accessories, you can make any plain black outfit pop with the right jewelry and accessories.