ANSWER: Firstly, Merry Meet friend! This is a tough situation, but it is especially so for someone young like yourself - and well done to you for knowing yourself enough to explore. Let’s explore some things to think about…….
Some prep work before you sit down to tell them might help it all go a little smoother and especially if your parents are as you say and possibly quite open minded about these things, being prepared for your conversation will help. Think about what it is you believe and want to explore. They will probably have questions around these topics and if you have well thought out answers for them then it will be a big help to your conversation.
Think about when you will tell them too. Its much better to know that you will be undisturbed and can focus on what you have to say than having to rush or getting interrupted half-way through. Tell them you want to talk about something important to you and try to remain calm and reasonable while you tell them and during the conversation after.
Unfortunately, even the most level-headed person can react badly when shocked by news they weren’t expecting, especially when that news is about their child no matter what the child’s age, so be prepared for any kind of initial reaction and to have to leave the conversation for another time when everyone has calmed down. Try not to talk about what your beliefs aren’t, so for example don’t say something about it not being devil worship because then the focus of the conversation will naturally fall more on unhealthy misconceptions rather than the actual path of witchcraft and what that means to you as a person.
I can’t tell you exactly what to say because I don't know you or your parents but I hope this advice at least helps. At some point or another most witches are in your situation and the best place to start is with the people who are most likely to love and support you no matter what. I truly hope all goes well for you.