How can a 13 year old girl make her room look gothic

If you are not allowed to repaint the walls, then I can suggest investing in some Victorian furnishings. Furniture with fancy patterns and unusual carvings make for a dark yet interesting statement. You can even paint it in a darker shade to suit your mood (though I do not recommend black, as it will block out the fancy patterns that you may want to keep). If furniture is too expensive, replace any brightly coloured curtains with sheers (tall version of a curtain) that have black and white print patterns. Change your table lamp (I highly recommend the Phillipe Starck Bougie lamp in black), and remove any personal items that may look childish or out of place.