Green Day are the perfect band for the beginner guitarist. Most of their song have a simple format with a good strong melody. This means that it doesn't take long before you get the song sounding right, and this is important for the development of any beginner. The better results you have when learning a song will reflect on how you approach the next.
Riff 1 is repeated for the intro and the verses. When playing, remember to start off slow and play each note at an even tempo. Once you get better then try to speed up to the original speed of the song. All notes are eighth notes in riff 1.
This is the only variation in the song, take note of which notes are quarter notes and which are eighths. (The first is a quarter note only equal to 1 beat)
This is the rhythm that accompanies riff 1, so if your practicing with someone else one person can be playing the riff while the other is strumming these chords. Take note of the "X" on the tab, this means to mute the strings. You'll find you may do this naturally as you change from one chord to the next. Start changing to the next chord on the 4th strum of every chord, this will give you a little more time to get the next chord correct.
This is the rhythm that accompanies riff 2. Take note of the rhythm pattern, it's a little more complex than the one above.