Travel back in time to 1995. Play Vampire: The Masquerade. Wear Marilyn Manson t-shirts.* Smoke clove cigarettes. Hang out in Hot Topic. Wear a pentacle, and correct anyone who calls it a pentagram. Watch The Crow, The Craft, and Lost Boys and never fucking shut up about them. Wear fishnet stockings on your arms. Get a trenchcoat in your favorite shade of black, with matching boots. Dye your hair black, or possibly ginger-red if female.
Even if you never land the girl, you'll be so goth your lawn cuts itself.
(* - yes, I know, he's industrial, not goth at all, as was understood in the 80s and 90s - that doesn't change what the actually existing goth kids did.)