Buy an acoustic guitar. You don't need anything fancy. Anything with six strings, the ability to produce sound along all sections of the neck and stay in tune for reasonable amounts of time will suffice. You're playing punk, after all, which isn't known for its formality.
Master open chords. Folk music is based on open-chord structures that resonate on an acoustic guitar. Most folk-punk acts play open chords rooted on the low E, A and D strings above the fifth fret.
Avoid power chords. Generations of punks limped along with sloppy power chords (which use the root note and the fifth note only). But without distortion and other effects to fatten them out, power chords usually sound thin and lifeless when played on an acoustic guitar.
Learn major key structures. Between folk and punk, there are no fancy music-theory arrangements going on. Both embrace the sounds produced by songs arranged in major keys with major-key solos usually rooted very high on the neck of the low E string.
Practice picking patterns. Punk guitar tends to emphasize the down stroke. But folk punk places equal emphasis on upstroke-picking that belies the style's Americana roots.
Study the masters. Bragg, DiFranco and The Knitters helped shape the idea of folk-punk and its sub-genre cousin, the country-flavored cow-punk. Picking patterns, key changes and song structures that define modern-day punk troubadours shine through most of their songs.