Whether you're into classic underground bands like Screeching Weasel or newer mainstream bands like Fall Out Boy, you can write a pop punk song similar to these bands. Writing a pop punk song isn't too difficult; all you need is three chords, a good melody, and a fast tempo, and you'll be ready to play a basement show (or an arena) in no time.
Write a verse. Write a simple three or four chord progression and palm mute power chords. One of the most popular pop punk progressions is the classic I-IV-V heard in songs like The Ramones' "Blitzkrieg Bop" and Green Day's "Church On Sunday."
Write a chorus. Write a simple chord progression and play open power chords on guitar. It can even be the same chords as the verse; just change the number of beats you play on a certain chord to change things up.
Write a bass line to the chords. The bass line can be as simple as the root notes to the song, or it can follow more of a melodic approach as Mike Dirnt of Green Day did on albums like "Dookie" and "Insomniac."
Write lyrics about love, boredom, alienation, drugs, school, or stuff that bothers you in general. Your lyrics can be risque as in Buzzcocks' "Orgasm Addict" or sweet as in MxPx's "Two Whole Years."
Write catchy vocal melodies. The word here is "pop" punk. Most pop punk songs are just pop songs sped up, so be sure your song is easy to sing along to.
Play it fast. Most pop punk songs are under three minutes. Write fast drum parts to keep the song moving along.