This is probably the most personal post you will ever see on here, and also one of the longest. Apologies for writing a whole book on the subject...
These are the most likely responses you'll get from writers. They are also 100% why I became a writer. But the "why" is easy. "How" is a lot harder.
I started writing my first romance novel when I was 12, on a family vacation to Virginia Beach. I was a lonely and weird child (I'm still weird, but not nearly as lonely now) and I had a hard time connecting with anyone. I took an omnibus edition of Sherlock Holmes with me (totally light beach reading for a pubescent girl), but what I really wanted was a love story. So I got some paper and started writing.
It was bad. Terribly bad. Looking at my spelling you would assume I was dyslexic. I basically took the plot of another book I'd read that year and stuck it on the beach was writing on. The characters were laughably Mary Sue-ish. I even named the hero after the boy I had a crush on. I gave up about 80 hand-written pages in (damn good for a 12 year old I think).
I wrote a tragi-comedy about a kid who goes swimming and is drowned by a ghost living in a lake. I wrote a few revenge stories about some bullies at school. I still have all of these, by the way. Hopefully in the future, when I am studied by scholars my juvenilia will be discussed at length. I wrote a couple things for school that were considered good enough to be read aloud at morning assembly. It was a discovery period. They were original stories that copied the styles of books I read. Lots of Christopher Pike-esque stuff full of murder and monsters. (He was my favorite.)
When my mom upgraded our family computer I got the old one for my room. That was exciting--a computer all to myself. Most of the kids I knew didn't have a family computer at all, let alone one of their very own. Who knows if I would have kept writing without it. I didn't learn how to spell until I had a computer in my room, that's for damn sure. I felt like a real writer behind that computer.
While at my computer I had to wear an ugly grey cardigan, stick pencils in my hair, and drink lots of tea. Because that's what writers did. I knew I was going to become rich and famous, and have an apartment in Manhattan with a white cat. I was going to publish my first book before the age of 23, because that was how old Christopher Pike was when he published his first book. (Reality: suburbs in Ohio writing in my kitchen, but I did have a white cat until she passed away recently.)
I learned how to write in high school writing Buffy fanfic. Say what you will about fanfiction, but when you're copying someone else's work you start paying very close attention to characters. You have to learn what makes them different from each other, their motivations, their secret pain. You want to get it just right. Plot also becomes a focus. Whatever you're writing has to be something that hasn't been done on the show before. You want to be original, to stick out from all of the other Buffy/Angel romance fests. For the record, the "Gem of Amarrah" story arc in season 4? I wrote it as fanfic back in season 2.
With fanfic it's important to get the tone right. With Buffy you need to be quirky and fun over darkness. In Harry Potter you need a level of juvenile angst--but not over the top like Twilight. In Jane Austen you better get the language right above all else. If you only write one genre your entire life, style isn't something you have to think about. But I like to jump around and fanfic became an invaluable teaching tool in learning different styles. Writing a story that sounded like the original was very important to me. Language is different. Tone changes.
I spent about six years total writing almost nothing but fanfic. Buffy, Harry Potter, Pirates of the Caribbean, and dozens of other books and movies. Then I discovered Nanowrimo. (That's National Novel Writing Month, in case you've been living under a rock.) I wrote a 35k word novella--the longest finished piece I had ever written. It was terrible. Naive and kind of obnoxious. I was a 19 year old mostly straight girl desperately in love with a lesbian at the time, so you can imagine the politically incorrect trainwreck that resulted.
Nanowrimo is great for learning how to write because it makes you write. It encourages having fun with your book, but it also encourages diligence. Finishing is so very, very important, and something not a lot of young or beginning writers manage. Hell, I've been at this for more than half my life and i still have more unfinished pieces than finished ones.
I published my first professional story at the age of 20 in an erotica anthology. I will not tell you where or under what name, because it is terrible. I was trying to write erotica when I'd only been having sex for a year and I had the emotional maturity of a potato. For a few months after selling the story I was elated, especially once I got my $50 check and 2 contributor's copies. I was a real writer. I'd made it.
Once the excitement wore off I read the book and hated every story in it. They were badly written. They were juvenile....and so was my story.
So after that I decided to "work on my writing" for a while before trying to publish anything again. I started reading about writing a lot. I got magazines, The Writer and Writer's Digest from the library by the armfuls. I learned that editing involved more than checking your spelling. I took literature classes, film criticism classes, and creative writing classes. The other students didn't seem to appreciate my work (there were an awful lot of Christians in my class) but the professor told me I would be in bookstores one day.
(Side note: Its amazing how much encouragement from a teacher can have an effect on a person, even as an adult. No matter how many people tell me how good my workis, my teachers' opinions have always mattered the most. If you are a teacher, no matter what you teach, if you see something special in a student make sure they know.)
I learned how much money a writer gets for a first advance. (Almost nothing.) And I learned how often those first books never sell. I learned about slush piles and agents and that sometimes your book sat with editors for months without a reply and they all hated simultaneous submissions. I got realistic. I kept writing, but publishing was no longer the pot of gold at the end of the rainbow.
Fast-forward, 2007. I mostly wrote for my classes (going to school part-time it took me 10 years to earn my degree) and still wrote fanfic. I discovered a new form of writing--online serials. It wasn't really new. Lot of books were written as serials in magazines as far back as the 1800's. Dickens and Sir Arthur Conan Doyle wrote novels this way. Stephen King used the same concept to write The Green Mile. (It was originally published as a 6 part novella series.)
These serials were like the long-form fanfics I loved to read and write, except they were originals written on blogs, and the authors had found a way to monetize them. Some only relied on ad revenue. Others would only post new chapters when donations were made. Some posted bonus chapters in exchange for donations. The authors interacted with the readers as they wrote the stories. All of this appealed to me. I wrote four of them between 2007 and 2010. I didn't make a lot of money doing this, around $600 total, give or take. Because of the method I wrote them in (mostly by the seat of my pants), the stories are all deeply flawed. But these stories were definite confidence boosters and were the first time I considered self-publishing a valid option.
Too bad I lost my boyfriend, my job, my house, and my cats in the years between 2008 and 2011. I got a little depressed. A lot depressed. I don't need to talk about it here, but it was really, really bad.
When life got a little less horrible I knew it was time to put away the fanfic and the serials and start becoming a writer "for real." I got a few shorts published in some anthologies, and 50 Shades of Grey showed me that there is a market for self-published porn. And here I am. I'm not making a lot of money, but I'm making some, and as long as I keep at it I will soon be making more.
My writing has changed a lot between that first published story ten years ago and today. Living as an adult for ten years has definitely shifted my outlook on love, relationships, and the world at large. I'm not writing silly adolescent fantasies anymore more. Maturity in writing comes not only from living to an older age, but from writing through that whole life as well. Learning, failing, succeeding, it makes a difference on the page.
The little girl I started out as, lonely on the beach, wrote a disaster of a book. But each year she got a little bit better. The differences between how I wrote at 20, and 25, and now in my 30's are amazing. Everyone's first book is a disaster, even if they become world-famous with it and make millions of dollars. It's a long journey from that first book to something truly great.
The more you write, the more your work changes. My books are different from what they used to be...imagine what they'll be in another ten years.