well legally your parents need to know and anyone who teaches you without their consent can face legal action. so i suggest you do some basic research then speak with your parents if you still feel strongly about it.
How can I practice Wicca in secret?
again, speak with your parents, but you can easily. meditation isn't exclusively Wiccan for example. you could say a prayer quietly to the Lord and Lady, observe the full moon and just reflect on the month. walks in nature and observation are another way to just connect with the world around you. you can cast spells, but it's not essential to Wicca [and circles can be done outside in the woods, and you don't need any materials for that either]
Do I need/ how can I get robes, wands, crystals, etc?
nope. it's all your personal connection. it's nice to have, but you can get creative [or simply use your finger instead of a wand or athame to direct energy] robes are nice, but again not essential [love to have one myself, and i've been studying over 10 years!] you can also take stuff form your pantry if you ever need supplies [a salt bath before a ritual to cleanse yourself of energy for example] candles and incense can be found at dollar stores [most things can be found in non-witchy shops if you really want tools] for books you can check out a big name book store [Canada has Chapters, i think US is Barnes and Noble? they have a spiritual section. or you could order in books]
Where can I keep hidden?
again, depends on the tools, a locked drawer for example, otherwise you can keep them hidden in plain sight [candles and incense can be displayed]
Can I be both christian and Wiccan?
*sigh* this ones kind of a grey area. by definition no. Wicca is a religion worshiping a female and male fertility deities. Christianity workships one God and his son [possible exception to Mary] so no, the two blend as well as cream and bricks. however, there are those who practice 'Christian Wicca' [the terms should be 'Christian Witchcraft' though] so technically there is a path that combines the two, but by the textbook definition, no. [but your path you decide]
How can I join?
Wicca has a year and a day ritual where you study for one year and one day, at the end you should have a strong understanding of the path and if you wish to join. you would then do a self dedication to the Lord and Lady and can be called 'Wiccan' [unless you join a coven, but at your age it's very unlikely]