How can you join wicca?

You can join anytime!

Technically as long as you worship the God and the Goddess and practice the craft you are Wiccan.

First off though it would be helpful to you if you buy a beginner Wiccan book. It's a great help, it tells you about initiation (a more formal way to become Wiccan), it gives you a list of basic supplies you need, tells you how to arrange your alter properly, and also gives you basic spells for beginners. Be sure to look at a local bookstores website and read descriptions of the books - or maybe try Amazon.com.