well since you're 15, you kind of have to tell your parent or guardian because they are responsible for you and are looking out for you. once you turn 18 [or whatever the age of consent is for your country] then you can do whatever and they have no say. until that time though, they need to know.
don't drop it on them and start an argument, won't end well. gather information, books, pamphlets, easiest would be print off some pages online and highlight the important points. then sit them down, ask them to hear you out completely before responding, then calmly explain to them what you believe. explain what your beliefs are, why you've converted, why you feel it's right for you and all you want is for them to support you. you haven't changed, the stigma and things you've heard/seen in movies are not true.
if they say yes, good for you, you can continue on. if they say no however, you have to respect their wishes, and stop practicing. if you're meant to be on this path, you can return when you're 18. until that time however, you can still study things like herbalism, history, mythology, crystals, yoga, meditation and a bunch of other things that on the surface don't seem like paganism, but do connect with the religion. spell casting is not why one should join a pagan faith, there's a lot of study from a lot of paths.