I'd assume tremolo picking is the same as (what I call) speed picking. Usually its a repeated hit on the same string at a rather fast speed. Its sort of like pinch harmonics; once you've learned how to do it its a piece of cake. I turn my pick so the edge hits the string. Seems to be faster that way...
And what songs exactly are you trying to learn?
You're an inspiration for birth control. - Duke Nukem.