How to Become a Great Writer in 10 Minutes: a Non-Comprehensive Guide
Your Microsoft Word program has a spellcheck program and some basic grammar knowledge (although once it did tell me to use the phrase "there am" in an essay...). Use them.
Eliminate adverbs. "The road to hell is paved with adverbs." ~Stephen King
Hear the dialogue in your head. Nobody says, "Let us go to the store, Irena, for there we can purchase milk and low-fat steak with my new credit card." Listen to what your character has to say rather than prompting your character to say whatever she needs to in order to keep the plot moving. Your characters are real people, not puppets.
Say to yourself: "In the beginning, there was (this). But then (this) happened. So (insert character name here) did (this). In the end, (this) is how things sorted themselves out." That's the only outline I have before I start writing anything. If an essay needs a thesis statement, this is your proverbial thesis.
Here is your proverbial character thesis: "(So-and-so) is a (adjective) (age) (noun) who wants (something)." And then, to tie this one into the previous one, you add, "so s/he (does this)." My favorite thing to tell my beta-ees is that every character needs to be ACTIVELY STRIVING after something.
(If you would like an example, here are my proverbial theses for my short story Hanged My Sorrows: Ray is a crotchety 80-year-old widower who wants to hold onto all his grudges. So he writes them on paper cranes and hangs them from his ceiling. In the beginning, there was an annoying cable guy that Ray wrote into a paper crane. But then it fell into a candle. So Ray tried to save all his paper cranes and beat the fire out. In the end, the fire consumed the old man and all his paper cranes. There, I just ruined the ending for you.)
You can (and should) use the story-thesis outline for individual scenes, too.
Cliches aren't bad. It's when you don't add to them that the problems start.
Don't worry about the theme of your story. It'll be there whether you want it or not. But if you start thinking about it, it gets heavy-handed and annoying - and then reading your story is more like reading The Book of Virtues.
Name your characters something reasonable. Leighton is not a reasonable name for somebody's grandma. Silver-Lee Joy Whistlewind is not a reasonable name for anyone. My name is THE most common first name for people of my gender and age, and I'm doing just fine.
You better believe I'm going to stop reading your story if you spend more than two sentences describing "the luscious golden hills of September as they sweep across the breast of the earth in a fiery explosion of orange and peace, and then a dove fluttered across the broad sapphire sky and landed upon the crest of the hill with delicate grace." I don't care. It's called "purple prose" and nobody's that fond of it. Please feel free to write poetry.
Describe your character's physical appearance as little as possible. To me it's like watching the movie version of your favorite book - no matter what actor it is, he just doesn't look like the character in your head. ("Katniss isn't skinny enough and Grover is too black and Edward is too ugly!") It alienates the reader.
There is nothing wrong with "he said" and "said Melinda." Your characters don't always need to exclaim, retort, cry, tremor, or grumble.
Just because Percy Jackson is sarcastic doesn't mean that your character has to be, too.
Don't start Chapter One with a, "Hi, my name is Geraldina, and I'm fourteen and I go to Hill Valley High School and blah blah blah blah blah blah..." Nor should you start with a, "I woke up and searched for something to wear to school." Nor should you start with a, "Five million years ago, a man named Jones took over the earth, and then a bunch of stuff happened (usually this goes on for a few thousand words), and now we all live in a big hole fourteen miles below the surface of the earth."
The main point is to start with action that is somehow specific to your character, so that we may learn about your character through that action.
Lastly, remember this: my title totally lied to you. You will never wake up one morning and be a good writer. You will never read any guide to writing and just *snap* become a good writer. These points were just some helpful bits of advice I, as someone who has been working on being a good writer for almost half of my (admittedly short) life, thought I could give you. As Ray Bradbury said, "You can't be anything that quickly."