Where to start with these things?

I'm 15 years old, I love movies and TV shows (well, certain movies and TV shows anyway).

Ideas for stories I like pop into my head and I try (key word) to put them into some kind of legible writing :)

Favorites: Doctor Who, Firefly/Serenity, Torchwood, Lost, Life on Mars/Ashes to Ashes, Stargate Universe