harmonizing chord progressions

ok my question, how do you know what chord progession you uses to harmonize with the rythem guitar, for like emo/alternative music

If you mean "scale" rather than "chord progression", then it depends on what chords you have. Check out the reverse scales link at the top of the page. Input your chords (by clicking on the neck) and it will give you a list of scales that you can use to play over the rhythm guitar.