Here are few few pieces of advice to motivate you while writing- fiction or nonfiction!
First and foremost, know your subject. If you are writing a story, know your characters. If you are writing an essay, research the topic. The more information you have, the clearer your topic becomes to explain or argue.
Second, be passionate! Whether you are writing about what on earth Heathcliffe's deal with Cathy was, the lifecycle of a frog, or why orange Starburst are better than yellow, you need to be invested in what you write. Sometimes you are given boring topics- regardless, try to put a bit of yourself in your writing. If you are bored writing it, your audience will be bored reading it. When all else fails, try to fall back on your personal beliefs and values for inspiration; for example, "According to three out of four students at Spring Hill Middle School, Minecraft is the best video game of this generation. That statistic rises to four out of four people if I include myself in that count."
Third, try to utilize interesting vocabulary! This can be manifested in different ways. You can use adjectives and spice up your nouns. You be more thorough in describing situations or scenarios. You can remove adverbs from your writing and replace them with strong verbs (such as changing "moved silently" to "stalked" or "tip-toed"). If a word is on the tip of your tongue or you can't think of a proper synonym, try doing a web search or using a thesaurus.
And finally, when all else fails, make a funny rough draft. Spend this draft whining about how useless you found Wuthering Heights, or about how much frogs creep you out. Get out all of your aggression and/or apathy while still staying on topic. Once that rough draft is done and out of your system, you can make your formal rough draft... and who knows, maybe a funny comment or two can migrate from your funny rough draft to your official one?