1)Is Wicca Safe? Aside from separating Traditional from Eclecticism- yes it is safe. Yes, I used a big word you're going to want to know.
2)What Supplies Would You Need? Technically nothing, but if you're interested in setting up an altar, you may need some physical representations of the elements.
3)Where can you buy supplies? For the most part, anywhere you can afford, and it doesn't have to be fancy.
4)What happens in the wicca circle? For a general idea, energy flows. You'd have to be more specific with this question.
Since you are 12, I'm going to let you know to make sure that you get your parent's permission to study witchcraft before you do anything. if they don't allow you to, it's not because they're being mean to you, following a religion is a serious decision to make that I wouldn't suggest you even try focusing on until 16, at best 18 years old.
If you're really interested and your parents allow you to, I'd suggest getting the book "Wicca for Beginners" by Thea Sabin and "Wicca for the Solitary Practitioner" by Scott Cunningham. You're going to want to read so you can make sure you know what you're doing before you actually begin dedicating yourself to the craft.
Witch, 5 years.