I've read a lot of advice on this and have put together all that I think is helpful on how to avoid looking like you woke up and went 'I'll be a goth today' without having any knowlage on how to dress and be goth
1) Don't act deppressed all the time. I'm sure you all already know this, but goths aren't deppressed, suicidel, self-harming people! Most goths are happier than people who are constantly pretending to be happy, because they acknowledge that life won't be one big happy time, so the sad times don't hit them as hard. Not that I'm saying all goths are happy, everyone's different.
2) Don't pile on the white make up and black clothes. Of course, you think goth, you think white faces and black clothes. But don't, please don't, paste whatever you use to make yourself paler messily over your face and put on any black clothes you find. If you like the pale look (like me :D) do it carefully. Pick a foundation a few shades lighter than your skin tone and blend it into your face and neck. Blend is the key word here. And as for clothes...just because it's black doesn't make it goth, and just because it's not black doesn't make it not goth. Look at clothes. Are they nice? Would they go with your wardrobe? Could you alter them to make them nicer? If yes to at least 2 out of 3 of these questions, buy it. If not, sorry, but it probably won't work. And jewel tones, such as deep red, are some of my favourite color clothes. These colors are also good for accents on clothes. You can go ice goth for a while and wear only white. Goth is about expressing yourself, not pulling on anything that's black. Although black is the main color of goth clothing...
3) Learn about the history of it. Goth started in the 70's. Learn about how it began, some of the original singers of goth music. The Ultimate Goth Guide has a lot of information on it if you look back through previous posts
4) Listen to the music. No, I don't mean Marilyn Manson. If you like him, fine, but he's not goth. Some of my favourite artists/bands are Emilie Autumn (I <3 you Emilie), Tori Amos and Love is Colder Than Death, but there are hundreds more out there. Again, The Ultimate Goth Guide has some artists to start you off (Guess which blog I recommend for everything? Correct, it is The Ultimate Goth Guide). But don't force yourself to listen to music if you don't like it. There are so many different genres of Goth music, if one isn't for you there's plenty more
Those are my four main points, but if anyones reading this, add more, because I know I've only covered a tiny amount. I may do a guest post soon by Juliet's Lace that's much more detailed on how to avoid the 'obvious newbie' look