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Wanna know the trick? You must read books and be influenced
Use descriptive words.
Ex) She was the prettiest girl I've ever seen. (Describing her face)
Even from a far distance, her sharp yet delicate features stood out from the rest. (Go on to describing your character's facial features such as a small pointed nose etc.)
Simple words don't make the content of your stories any less interesting or anything, it's just the matter of how you write it. Just like AdequateDisclosure said, you need to read certain books and be influenced by a certain writing style before you can be comfortable with writing your own and liking it.
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While reading other people's work is important, really making your writing an expression of yourself has nothing to do with following someone else's example. Your real expression will come when you understand the methods of others and yet prefer to say what you mean in your own way - not in mimicry of what you like, but as an act of self-expression.
If you want a better vocabulary, read classics and figure out what the words mean through context clues. Much of our present-day collective vocabulary has been reduced to a 6th grade level and even good modern writers don't make prolific use of diverse word choices. This is why journalists are mandated to write for an 8th grade reading level. Realize that vocabulary is all about precision and that having access to the correct word in the correct context is the foundation of eloquence.