You don't want to paint your walls a dark colour, it'll make them look really small.
What I recommend is Halloween type decorations that look quite spooky or creepy and ornaments like statues of Gothic angels.
You could settle for a Gothic designed bed cover, black curtains and a rug etc, it'll give your room a genuine dark look.
That's what I've done with mine, but with band posters over one wall.
Some of these are do-it-yourself ideas:
http://media-cache-ak0.pinimg.com/origin...
https://www.etsy.com/listing/98492689/bu...
http://media-cache-ak0.pinimg.com/origin...
http://media-cache-ec0.pinimg.com/origin...
https://www.etsy.com/listing/59640957/3d...
I'd like to decorate my room Gothic but we're poor, I can't paint my walls, and I share a room with my sister. Please, ant ideas?