Very cool, slant, very cool.
Good Riddance was one of the first songs I learned to play. I was taught by a bunch of stoners at a local guitar shop.
If, however, that's not your thing (
), whenever I look for tab, I tend to visit
Olga.net,
guitartabs.cc, and
guitarnotes.com. Olga has just about everything, guitartabs lets the users rate the tabs, so that helps in finding a decent one and guitarnotes has a lot of the older stuff. There are tons of pretty good tab sites out there, but if all else fails a quick google search usually turns up a pretty decent fan site that usually has pretty accurate work.
David Hodge also has some killer stuff to read through at guitarnoise.com. I don't think any of Green Day's stuff is there, but his articles are definately worth a read through and the forums are a veritable treasure trove of information.
As for power chords...whew. To play them 'right' there are so many things to consider: finger strength & length, the kind of strings, how much 'play' is in the strings themselves, how high (or low) the strings set above the fretboard, etc, etc... It took me a while to get them and I still have problems with them five years later. I have short fingers, so I tend to cheat them (play only the top 4 strings) when I can get away with it.