For many Pagans and Wiccans, the decision to come out of the broom closet is a huge one. After all, once you're out, you can't really go back in. When you feel like it's time to come out to one of your friends, it can be even harder -- what if they dump you for not believing the same thing anymore?
Before you actually have the Big Conversation, think about what you're going to say. As silly as this sounds, know what you believe. After all, if your friend asks you questions, you better be able to answer them if you want to be taken seriously. Make sure you've done your homework beforehand. She may want to know what you believe about God, reincarnation, spell work, or even if you hate Christianity now that you're Wiccan. Have an honest answer ready.
Make sure you focus on what your belief system is, rather than what it isn't. If you start the conversation with, "Now, it's not devil worship…" then all your friend will hear is the "devil" part, and they'll start worrying. You may even want to recommend a book for her to read so she can understand Wicca and Paganism a little better. There's a great book called When Someone You Love is Wiccan. Although it's aimed at Christian parents of teens, it also does a nice job of breaking down modern Pagan beliefs into a Q&A format that's easy for non-Pagans to understand.
If you're not sure you can have a direct conversation with your friend about this just yet, try testing the waters a bit, and come out gradually. You could start by wearing a piece of religious jewelry and seeing if your friend notices it. When she asks, you can explain, "This is a symbol of my faith, and it means [whatever]."
You may find that your friend is confused by this choice you've made, or she may even be angry. She may feel hurt that you haven't talked to them about it before, or even a little betrayed that you are leaving behind a faith that the two of you once shared. Reassure her that you've not made this decision lightly -- and that despite the differences in your beliefs, you still love her as you always have. Be sure you answer her questions honestly, and make sure she understands that just because you're no longer part of their religion doesn’t mean you no longer want to be friends. If she's truly your friend, eventually she'll come around and be happy that you're happy.
The down side of this, of course, is that she could well drop you like a hot potato. If this happens, there won't be much you can do about it, other than be thankful for the few years you had as friends together. However, many people find it is more important to be honest about their beliefs than to hide, and maintain a friendship based on a lie.