Hi, I’m 14 and I love writing. I started writing when I was 11 but no one would read them because they were long and boring. I used to write fairy tales when I was young and people said that I was imaginative but my vocabulary was not that good. I tried improving my vocabulary by reading other books but it didn't work because I would forget them. I want to write fiction. I need some help to make my stories interesting.
Having a bigger vocabulary is certainly useful - like having a lot of different tools in your toolbox. The more words you know, the greater your chances of finding the right word to express what you want to express.
That said, it's also not the be-all end-all. The two books that have had the biggest impact on the English language were the King James Bible and the Collected Works of William Shakespeare (both from the Elizabethan period). Shakespeare had a vocabulary of 17,000-20,000 words, many of which he invented himself. The team that translated the Bible used only 6,000 different words. So size of vocabulary is not the way to measure quality of writing.
Reading a lot certainly helps you immensely when it comes to writing, by exposing you to many other stories and writing styles. However, if you want to kick it up a notch, I would recommend two other exercises. You may find them boring (or not), but they really work.
1. Memorization. Learning poems by heart, or speeches (as in drama) is a fantastic way to get words and ideas into your subconscious. When I was younger than you, I started memorizing poems from The Lord of the Rings
and other books I liked. Later I moved on to Shakespeare and performed in a lot of plays. The plays especially were fun and all that memorization has helped my writing.
2. Copying. This one is less fun, but also very effective. Whenever you find a writer whose style you really like, try copying out a chapter or two from one of their books. You can do this longhand or via keyboard. This will force you to slow down and really concentrate on the writer's style. Do it regularly, and your command of language will really increase.