There's no faster route to making an impressive vocabulary word unimpressive than to use it in a way that's not quite right, because you just learned it five minutes ago and are working solely off the dictionary definition. I knew someone in high school who had to write a sentence using the word 'deter,' which means, 'to prevent or block.' His sentence was: " 'Stop or I'll deter you!' the girl yelled." Which tells the teacher nothing except that he just read the definition of 'deter.'
Look at it this way: a good vocabulary is like a good wardrobe: it takes time to accumulate. You can't just rush out to the store and fix a sloppy, unflattering wardrobe in one go, and likewise, it takes some thought and personal taste to improve your vocabulary. In short: read. Read diverse things -- novels, magazines,blogs. Watch good movies and TV -- there's very good dialogue there, too.
In addition: I don't wish to offend Stripes, below, but I disagree strongly with the advice she gave you. It runs counter to what I've heard every English teacher and editor tell students, from Strunk and White on down. Using the passive voice drains a sentence of its vitality. Which sounds better to you: 'I jumped off the bridge' or 'The bridge was jumped off by me'? Active voice is clear, direct and engaging. My first editor changed all my passive-voice constructions and told me, 'No, don't tell me such-and-such was done; show me people doing things' and I immediately got his point. That's what the active voice does. You can't make your prose better by making your sentences more wordy. Make your point and move on.
Sorry, I know this doesn't help you if you've got an assignment due shortly. But it'll help you throughout the rest of your academic life. Trust me.