1) Just because something is black, that does not make it Goth.
2) Try and get inspiration from the make-up of some Gothic icons - don't try and 'just make it up' yet!
3) Don't take every opportunity possible to rail against the failings of the world.
Also, being angry at the world is not a prerequisite to being Goth.
4) Try to listen to a wider variety of music genres.
I was into Marilyn Manson, Within Temptation, Evanescence, Nightwish... and pretty much only Marilyn Manson, Within Temptation, Evanescence and Nightwish. I was very narrow in my tastes, and oblivious to how qualities I liked (and still like in music) aren't genre dependent; right now I love Hozier's "Take Me To Church" and Coldplay's "Cemeteries of London" - not exactly Siouxsie and the Banshees or Joy Division!
I went through a phase of almost ignoring the classical genres I actually really like because they didn't seem Goth enough... oh, if only I had put together then my playlists like the "Gothic Classical: Dinner Party" one I put together a short while back - excerpts of Léon Boëllmann's "Suite Gothique", plenty of Arvo Pärt's works ('De Profundis' is a masterpiece!), the absolutely divine and heavenly "In Paradisum" by Gabriel Fauré from his Requiem mass... I put together over 2 hours of amazing classical music with Gothic atmosphere very rapidly. If i wanted to put together something all dark and stormy but classical, I'm sure I could easily look to lots of Liszt (Totentanz?), 'Night on Bald Mountain' by Mussorgsky or a whole heap of Sturm und Drang period music (Beethoven!).
5) Don't try and deliberately be "shocking" as a method of rebellion.
I was a bit antagonistic and argumentative as a teen - I tried to be deliberately "scary", to mention witchcraft in a way that was probably dishonouring my religion, and make inappropriate criticisms of the faith and institutions I was surrounded by. I was lashing out at what I felt to be a very constrictive set of circumstances, but all I was doing was provoking people in a way that was counterproductive to my interests, and giving Goth and Wicca a bad name in the process. I was a very badly behaved babybat in this respect, and I am glad I did all this before social media really took off, so my impact was geographically and socially limited. I was young, emotionally immature for my physical years, dealing with partially treated mental health issues and very, very angry at the world, which to some extent explains some of my behaviour, but does not excuse it. I felt at odds with the "establishment" world, and felt like I was being pressured to conform to a set of standards and expectations that I had no interest in (and still don't) and that as I was heading to the age where I was expected to choose a career and what university I was to attend, that my whole life was going to be shaped by these pressures from my family and those around me, to become something "sensible" and "respectable" and most of all "normal" - and as I felt powerless to take any real control over my life, I decided to just try and antagonise all those who I perceived as trying to control me. All I achieved was to create more battles for myself, more unnecessary struggles in a life already full of unavoidable struggles; I made life considerably harder for myself.
My reaction to what I saw as "being forced to be what I'm not" was to try and deliberately be the opposite of everything expected of me. Most of these opposites were all exaggerations of myself, pushed to extremes in order to be that antithesis; I was really just being childish and contrary, these things were not who I really was. If I was able to go back and tell my teenage self some advice, I would say that I should definitely remain true to myself, but I should not react to the pressures to conform by deliberately being the antithesis of everything they wanted, because that was NOT being true to myself. With time, I learnt that I was not always going to be under the control of others, and now I am very keen to be mistress of my own life, and to be in control of the various aspects of how I live. By the time I was 19, I ended up with a part-time job, doing creative things at college (after I had to leave school a year early due to health reasons and then take time out to recover), and I am still Goth, still alternative, still proudly freaky and very creative over 10 years later. Some of this happened 13, 14 years ago now! I have not had to buckle to 'the system', I have not been forced to conform, and I have learnt that I do not have to live the life that other people felt was best for me.
Hindsight is 20/20. I made my mistakes, so now you don't have to!