You might want to start with the thread on Resources for Beginners at http://www.spellsofmagic.com/read_post.html?post=500751 And also check out other threads at the beginning of the Wicca forum.
It's important to understand that Wicca is not Celtic in origin. That being said, it is possible to practice Wicca while looking to some of the Gods and Goddesses of the Celts in your rituals. Generally it is this combination that is referred to as "Celtic Wicca". A couple of links you might want to look at include: