really suckie situation, but you have to tell them. they're responsible for you, and as bad as this might feel, you need to tell them. once you turn 18 you can do whatever you want, but until said time, they're responsible.
now, what you should do is find as much information about magic [and/or whatever religious path you follow if any] when you feel the time is right, sit them down and ask to explain yourself. calmly explain your belief, stress it's not evil, that there's no devil or evil being, it comes down to a person's intent. tell them why you choose to practice, and what you get out of it. try to answer all their questions, and provide sites/book sources for them to read themselves.
granted, not all parents are as understanding as mine is, i get that, but hopefully they'll listen to everything. try not to fight, or argue, you might get upset if they say no, but try not to yell and scream. if they do say no, accept it. paganism teaches if you're meant to follow this path, you will return to it. 18 might feel a long ways away, but in the grand scheme of things it really isn't. on the upside though, you can still study things that don't seem to be involved with magic.
if your parents say no, you can study history, herbalism, cooking, candle-making, stones and crystals, genealogy, you could start a garden, take up sewing, meditate, learn about your chakra's, and many other things. they seemingly don't have much to do with magic, but they're things that you'll need to know and work with when casting spells. you're not casting, but you can still do things that are similar.