Gothic characters just need to wear dark clothing - primarily black. You can add a pop of red or purple, as well. 
Ruffles, lace, and high necks. Flowing sleeves, skirts, capes. Most of the Gothic attire will have a Victorian inspiration, so vampire costumes can work for Gothic characters of all kinds - just don't wear the teeth or carry a coffin purse if you're not a vampire. 
Accessories should be silver or pewter featuring skulls, crosses, bats or other Gothic motifs. Chokers, especially velvet or lace are iconic. 
Footwear - boots are a mainstay, especially combat, platform, or Victorian lace-up boots. 
Hair: black hair is the most common, but any color can be made to look Gothic with the right style. 
Remember, Gothic fashion is diverse, and there's a lot of room for personal expression. Feel free to mix elements from different Gothic subgenres, like romantic, industrial, or cybergoth, to create a look that's uniquely yours. The most important aspect is to wear what you feel comfortable and confident in, and what best expresses your individual take on the Gothic aesthetic.