Getting started writing isn't rocket science. The late Marion Zimmer Bradley defined it quite succinctly: "Apply the seat of the pants to the seat of the typewriter chair and stay there until you get results." There's really not much more than that.
Well, there's not much more to it than that unless you want to write stories that sell. Even here, it's hard to determine any hard and fast rules. It seems that for any rule one might name there's somebody out there breaking it successfully. Still, there are some things that can help.
1) If you want to write, you have to be a reader. Read voraciously. If you want to write in a particular genre--science fiction, fantasy, mysteries, westerns, romance, whatever--then read that genre. But don't read only that genre. Stretch your horizons. Read classics. Read non-fiction. Want to write Science Fiction? Read Romance. Want to write Romance? Read Westerns. Now, in addition to the reading, think about what you've read. What did you like about it? What did you dislike? If you liked some parts and disliked others, try to figure out why. What was different?
What you're doing here is learning what goes into a successful story, what works for you and what doesn't. You can read reviews to get an idea of what worked and didn't work for others, and that can be valuable, but in the end it comes down to what works for you and no one else.
2) Get out and meet people. Watch them. Talk to them. Take notes, if only mental ones. In the end, stories are about people. All these interactions with people in the real world--good, bad, or indifferent--are ingredients that will go into your characters in stories. If your only source for characters comes from reading other people's stories your characters will seem derivative and shallow. They will seem that way because they are derivative and shallow. So bring some real people into the mix.
I'm not a big fan of basing individual characters on individual real people but taking a bit from here and a bit from there and one can build interesting characters that will seem real to the readers. And that's the goal. They don't have to be real. They have to seem real.
3) Above all, write. Write character sketches, story ideas, complete stories. Write write write. Then take what you've learned from the reading and look at it again. What does and doesn't work in what you're writing? Something not working? Can you fix it? If so, try that. If not, set it aside (after all, there may be something you can mine out of that later) and try something else.
A "truism" in writing is that one has to write about a million words worth of crap in simply learning the craft. Now, there are some exceptions to that rule (you know who you are; yes you do) but that's a pretty good rule of thumb. While there is a "talent" aspect to writing and storytelling they are also crafts that can be learned. But here's the thing, you can't just write any crap. You have to be trying your absolute best, writing the best crap you know how to write.
I started writing sometime in grade school. I was, at that time, a big reader mostly of science fiction. I was a big "space buff" and was writing mainly to get more stuff to read. During those years I wrote a lot of story beginnings but very few if any complete stories.
Then came 1977 and That Movie. I loved the movie. It inspired me to write a screenplay of my own. It inspired me to finish a screenplay of my own. Now, my screenplay was a cheap ripoff of Star Wars. It was a bad cheap ripoff of Star Wars. I mean this thing was seriously bad. I live in fear that somebody, somewhere, might turn up the only copy of the manuscript (written by hand on notebook paper) and threaten to release it to the world if I don't pay blackmail money. Bad. But, it was nevertheless an important turning point. It was the first piece of any length that I had finished. I could finish a story.
And so my next big story was a novel written in study hall at school. It too, was very bad. Hackneyed, cliched, and implausible. Oh, and the main character was a complete Mary Sue (Marty Stu?). But, again, I demonstrated that I could complete a work of significant length.
Shortly after that I joined the Air Force and played around with writing a bit more. I had discovered SF magazines and realized that people paid money for short work, including short work from new authors. And so I began writing short fiction in earnest. Most of my work in this period was still quite derivative I wasn't having any success. Writing was still an occasional thing with me but I'd write stories on my computer (Apple IIe), take the printed copy (9 pin dot matrix) over to the library to retype for submission, and send them out to collect rejection letters.
This was a very frustrating period but, without realizing it, I was learning the basics of the craft and the stories were getting better. It was in 1991 when I made my first sales. (More on that another time.)