Have you tried explaining exactly what it is that tarot cards are? I find that a rational explanation helps. Try this:
Tarot cards are a tool. They are not associated with any particular faith. They are a tool, and nothing more. The subconscious mind has access to information that the conscious mind does not. The shuffling of cards allows for a random display of symbolism that the subconscious can then interpret in regards to the situation.
Try not telling your parents you're into "the occult". "The occult" is a scary phrase, which has been demonized by Hollywood through such wonderful movies as "The Craft".
What else are you trying to do? Are you experimenting with your faith as well? Or do you still consider yourself a Christian? It's fine if you do. There are PLENTY of Christians who read tarot cards and tell fortunes. The only time I ever had my palm read, it was in a room that was STUFFED full of Catholic icons and imagery. She actually had me place my hand on an open bible.
A good way of approaching the subject could also be to say that you're interested in learning more about things that you've been experiencing.
But just for the record, I'm going to say it again: Tarot does not have any specific religious affiliation (depending on the deck you use). There are hundreds of decks, each with their own particular meanings. Reading tarot is not going to bring spirits or demons into your house. You will not be consorting with the devil, or with god for that matter: only with your subconscious. Good luck! Feel free to email me if you have more questions/need clarification on anything.