I used the above site for reading the Avesta, but I glanced at the one below as well.
This article presented a portrait of modern Zoroastrianism, which was informative and poignantly presented. Zoroastrianism is currently in the decline, as this is a fairly insular and exclusive religion. Though their scripts do not explicitly forbid it, many Zoroastrian priests do not accept converts or the children of a mixed marriage as Zoroastrians. Modern Zoroastrian communities are thinly dispersed, all over the globe, united by the basic tenet ” good thoughts, good words, good deeds.”
This site has a pretty barebones description of Zoroastrianism, with a few interesting facts. It is definitely a place that you can start trying to figure out what Zoroastrianism is.
Most importantly, this site has information on many religions, hot button issues, and other things which you may need to know in order to better understand a wide variety of religions.