Just use your knowledge of theory and play stuff. If you like the way it sounds, then present it to the band. You'd be surprised how fast songs evolve when you have one riff to start off with.
As for chord progressions, see what sounds good to you. There are some really wierd chord progressions out there that sound susprisingly good, and you don't want to recycle the same old I-IV-V-I progression over and over again. Experiment, and create something new.
For punk music, there is usually an intro (which is often the chorus riff as well), a verse riff, and some kind of bridge. This is a simplistic way of looking at it, but if you figure out a basic structure for the song, then you can create riffs, and for a basic punk song, you might need 3 riffs tops.
Also, try to just create music in your head. Like whenever I'm bored (or in class... haha) I just start "composing" in my head. When I get home, I play the riffs on my bass, and maybe record them if I like them. Try to capture the moment, and not forget the good riffs.
Plus let's face it, punk music isn't particularly ground breaking riff or chord-wise. It's usually pretty simple, but sounds good. So keep that in mind.
Like on my website, I made up all of those songs (except one cover). Make something up, and the rest will come to you.
