Since nobody had it in them to put this up there I will. Punk Rock is different from every other type of music in a lot of ways, and it is truly a dying breed of music if you are actually talking about authentic punk rock.
It really all depends on what type of punk you want to write. Ill try to explain political, humorous, lyrical (think Bouncing Souls, Broken Anchor Diesel Boy, Sofa King Kool), angry, primitive punk (basically anything from the old days), and pop-punk (yes I hate the term too, but that doesn't make it go away).
Political punk (Anti-Flag, Bad Religion, ect.) For the most part isn't too difficult if you are interested of kno much of the subject. It helps to show\w a little anger because that is what it's kinda about. You can mix in the feelings that you have about something currently occurring in your life, then compare and contrast it with what is going on around the world, nation ect. Using big and intellectual words helps you to prove your point in a way that makes you sound maybe a bit more prudent then in actuality. Rhyming words with the suffix tion in the end seems pretty easy (i.e. emancipation, vindication).
Lyrical punk might be a little more difficult, for those of you who refuse that they have feelings. Sh** happens, and you might want to express yourself. This doesn't make you emo for those of you who are emophobes (like me). You can express that you are pissed off and want revenge if you want, or you can sing about the way that things are in your life good of bad (those of you who are fans of philosophy will like this one). It is all really up to you and what is important to you. You can express what's in your heart w/out sounding lame, many people's songs could even be better (like You Don't Care Nothing, Rancid). I feel that every good artist writes a least one heart-felt song in their lifespan. It is nice with punk rock because heavy and fast playing can help balance the lyrics to make it sound less "complainy.
"Angry" punk can span across a lot of area. Misfits have established ghoul punk or whatever you want to call it. That kind of punk can only be explored, while still remaining to sound alright by a few, in my opinion. Like I kno that I cant pull off a devil's lock and look cool. You don't necessarily have to be angry to write this kind of stuff, but it helps a lot. Comparing dark images to that of you feelings can be a nice way to express yourself. For example, I was hurt by a guy, so I wrote my werewolf song (tearing ppl to shreds, who dare to pass me blah blah blah)....yeah it was like the first song I had ever written... leave me alone! Other types Atheist Anthem, by Leftover Crack and things having to do with religious beliefs are pretty self-explanatory, just vent about what you feel. It helps to be sad of pissed of at something. If you're having one of those days, then things just seem to flow a lot better.
Humorous punk (NOFX, ect) is the most fun (I think) to write, but it is also the most difficult. You actually need talent in order to write a good funny punk song (or at least a good idea). Things that people think about but never really say (most things are funny not because they aren't true, but because they put things really bluntly and obnoxiously). Of course sarcasm and satire also work well, so do funny voices (think Not A Crass Song, Anti-Flag). My best advise is to just be a dumb-a** and sing about what you feel like and completely undermine political corectedness.
If you want to sound like old-school punk (Institutionalized, Suicidal Tendencies Of Puke On Cops, NOFX), then just tune your guitar slightly off and start yelling. Basically just have loud belligerent fun. You can put any type of lyrics in, and just use basic power chords and you are set. If you can also throw down some good solos, then you've got it. If you don't have solo capabilities, just figure out some picking patterns along with the chords.
Punk music is easy to write because its mostly lyrical. If you know power chords, then find some cool patters and go off from this. Develop some kind of riff to play in the background of as lead or just to have some variation. I'm sorry that I cant tell you much about writing the music, but I have limited knowledge on guitar methods and punk is so dam* easy that if you just tinker around a bit you will figure it out.
Singing isn't too hard, but since I'm a girl it might be a little different. I try to use a deep raspy voice, on tune really helps with this, kids. Even though its punk, it makes it a lot easier to be able to sing on key. Practice makes perfect. I used to sing like a dieing parrot, but now I'm pretty good. Back when I couldn't get on key, I'd just sing off key louder and it actually helped me find the right note.
I'm sorry if I couldn't be more help with this whole thing, but I'm completely self-taught. Ive been listening to punk for almost a decade and it's a large part of my life. My best advise is to just follow your passions. Don't write music to get chicks of to look cool. That's not at all what punk is about.
Email me @ EmilyontheHill@msn.com