Write what you know
Once upon a time--when dinosaurs roamed the earth and I was still in high school--I had a wonderful English teacher who treated his students as peers and insisted that we all call him by his first name; his enthusiasm for literature and drama was outright contagious. He convinced me to read outside my preferred genres and he pushed me to write, write and write some more. A tremendously harsh critic, he somehow managed to be supportive even while delivering the most ruthless dissections of my prose. I was a bitter and catastrophically depressed teenager who approached each school day with apprehensive dread, but for his classes I held a genuine anticipation.
Until one lecture when he insisted that creating truly powerful fiction required that we 'write what we know', and all my enthusiasm burned away into ash.
At the time, I thought that he meant that our prose should be limited to our experiences and our areas of expertise. I couldn't imagine any subject less interesting or worthy of consideration. How could the experiences of a morose, sheltered and awkward kid be relevant to the life of an inhuman denizens of a fantasy dungeon? I wanted to write about dragons and laser pistols, camaraderie and adventure!
I occasionally wish for a time-machine so that I could leap across the years and smack my younger self on the back of the head. Because the truth is...every experience is something you can learn from. I may not have ever soared above a battlefield then folded my wings to drop into combat like the gryphon protagonist from one of my short stories...but I knew the feel of wind against my face and could add that sensation to describe my gryphon's flight. I knew what it looked like when a hawk stooped towards its prey. I knew what anger felt like, and fear, and hope, and sadness.
To 'write what you know' doesn't mean to write about yourself. It means to use your personal experiences to lend the power of authenticity of your prose.
There is a secondary meaning as well, and it is one that I try to take to heart more as an adult author: Research, knowledge and the acquisition of new sensory memories can make your writing more compelling. It's tempting to feel content that having swung a baseball bat is sufficient experience to write a scene in which an armored knight wields a mace, and it is definitely true that being able to evoke up the memory of how your grip strained or how your shoulder shook at the moment of impact is important. But spending time researching how maces were actually used historically can help create a more powerful scene. Look up how much real maces weighed. Research the kinds of wounds that a mace caused. If you can, make a mace and create new sensory memories by beating up an old tire. Interview experts and NEVER rely on anything you saw in a Hollywood blockbuster movie, because Hollywood is a lying liar who lies.
Try new things! Get your hands dirty in the garden, take a lesson in welding, bungee jump, hang-glide. Eat exotic foods and learn to mix cocktails. Live.
So, my advice to an aspiring author is this: Write what you know because you know much more than you think. And never, ever stop learning...because who knows what you're going to want to write about tomorrow?