Adding to what Uzuki wrote, I'd recommend you to create an original work. When you write a fanfic, you're using already existing characters, places, and all the imaterial stuff related to them, like relations between them, their background stories, etc. You won't be putting your brain to work much since you won't be effectively 100% creating something - you'll only be using premade material. But when you decide to create and original work, that's when all the magic begins. You'll discover your own characters, try to realise what they'd do in some specific situation, ask yourself what made them be who they are... You'll face many challenges and you'll need to overcome them. And when you do it, you'll be there, creating something with your own mark, with your own style. You'll be proud and will definitely have all the right to say "hey, I did this!"
Nothing against fanfiction, tho, really. But only working with them is like wanting to be an illustrator and only working with other peoples drawings. Learn to make yours! Just do it!