The best place to start if you want to know about gothic fashion is the 
alt.gothic.fashion FAQ <http://www.ravenmoon.us/agf/>.  It contains all 
manner of information on where to get goth clothes, how to paint your 
leather jacket, how to have more vibrant and impressive hair than the 
rest of your peers, etc.

A good starting point on the Web for goth and cyber clothing is Batgear 
Beyond <http://www.batgearbeyond.com/>.  It's like a version of Yahoo!  
dedicated just to goth fashion.

Building up a large goth wardrobe takes time and imagination.  There are 
many different styles of goth fashion, ranging from deathrock 
(black-clad punk), to vintage and Victoriana, to futuristic and cyber 
clubwear.  The trick is to find a look which suits you, and then to 
tailor it into something unique; you don't want to appear identical to 
the person sitting next to you!

Specialist goth clothing stores can be fantastic if you're able to get 
to them, but you shouldn't ignore high street clothing stores, 
especially during the after-Christmas sales.  Interesting items and 
accessories can turn up in the least likely places.  Beyond that, there 
are second-hand shops, antique clothing specialists, fetishwear 
suppliers, army surplus stores, and more, all of which may or may not 
have something that fits into your personal look.

If you're new to the scene, and are unsure of what to wear to a goth 
night, often the best advice is just to turn up in "normal" black 
clothes (T-shirt, jeans, dress, etc.).  A safe minimalist approach is 
preferable to a gothic fashion disaster, and still shows you've put some 
effort into your appearance.  This is usually enough to satisfy a 
"gothic dress code," though it would be advisable to double-check for 
large events.