Run! Run as fast as you can! Find overalls and a broom! Borrow money for trade school! Make a halfway decent LinkedIn profile! Learn to knit! Anything! Anything other than this! Fucking run!
Now that I've got that out of my system...
Don't be shitty is a good place to start. This is accomplished by straight up not being shitty. Here's how you figure this out. Can you write good? Did you realize that should have been 'can you write well?' You're shitty.
No, no, no, but seriously...
Writing for most people is super fucking hard. So you're likely shitty. Give up. Maybe even run.
Have we got that our of our system? Do you want the real skinny?
It's not that hard. Just do it. If you suck, you'll get better. Here are some tips that can help you get started.
1. Finish things. Don't start shit and not finish shit. That's dumb. You have to learn how to finish.
2. Once finished, get opinions. It would be nice to know in what way you're shitty. That's how you learn. Make sure to take it that way as best you can. Of course it sucks when someone thinks you're shitty, but just accept that's going to happen. A lot. If one out of a thousand people like what you do, you'll be rich and famous.
3. Fill your head with things you need. Obviously, you have to be a reader to be a writer, but you can also read books that specifically help you write. You need a style guide, first of all, (The Elements of Style by Strunk and White is the de facto best option) and probably a book like Stephen King's On Writing to get some honest perspective.
4. Create good habits. Writing is work. Hard work. Get ready to institute some gnarly structure in your life. It's very difficult and extremely rare for writers to write 'whenever they want' or 'whenever inspiration strikes' or 'whenever the muse arrives'. Toss the 'whenevers', sit the fuck down, and write.
5. Set realistic goals. And I do mean realistic. If you try to write two thousand words a day (like Stephen King) you may find yourself weeping in the corner after six days of definitive failure. I suggest a grace period of a few weeks wherein you discover what you can comfortably accomplish. If that's sixteen words a day, so be it. Lock that habit it, then grow from there. Also, do a little math. If you write say, a conservative five hundred words a day, you'll have a book in no time. Which leads me to...
6. Take baby steps. There's a lot to learn, some of which is almost impossible to describe. How do you cast aspersion on a character? Right? So here's the trick. Take it slow and steady. You are the tortoise, with glasses on and a will to share your creativity. If you want to write a novel, the shortest length acceptable is about 60,000 words. At five hundred words a day, that's only four months. Not bad, tortoise. Not bad.
7. Do you. This rule pretty much contradicts most of what I just said. You have to be you. After all, unless you're writing a technical manual for vacuum cleaners, this is art. Embrace that! Make it a capital word. Art! You are now an artist and you have to let the inner beast out to roam free. This is going to be different for every person, but here's the way I do it, to give you at least one example.
Me: I write linear, no jumping around. I write characters first, and they tell me the story. I don't shy away from things because they might not be 'marketable'. I always make the scarier choice. I accept that I can be considered niche. I understand that rejection is commonplace and doesn't mean I suck. Some days I think I suck anyway and I allow it to motivate me to get better.
(Also, some days I'm in the corner with you, crying and eating worms because nobody loves me. That's okay, too.)
8. Don't try to show off. You probably have some skills, or you wouldn't be considering this pursuit. You don't need to prove to people that you're good. You're a storyteller, not Mark Twain (or whoever your Mark Twain is). Tell the story. Use the words that help tell the story. You don't need to impress people with the way you write, not yet. You'll develop your own Voice through repetition. It will come, you can't force it. And here's a bonus tip: Ignore the old adage 'write what you know'. That's great for some genres, but you have to trust yourself as a creative. I've never been to Saturn, but I can write the shit out of a story that's set there.
9. You will suck. Man, you're going to be so shitty at first. Here's the thing, though: you won't even know. You'll fear it, and others may say it, but you won't know how shitty you are now until years later. But that's cool, 'cause that's how life works. You're always learning. It's a good thing that in five years you'll see the garbage you used to produce. That's how you'll get good enough in the long term. In the short, let out the beast, edit with as much brutality as you can muster, and gather a few trusted readers to help you out. In no time you'll be 'not shitty'.
Short Answer: Huh. I had some things to say about this. Cool. Maybe I'm not shitty, now.
Note: Also, on the whole 'write what you know front', you can pretty much learn anything in ten minutes on the internet, so go for it. You don't have to do days of study anymore. I often stop in the middle of what I'm doing, go online, and learn the name for that thing. It works just fine. 'Write what you know' does have merit, but if you don't inherently understand it, don't worry. If you do, apply as necessary. I pretty much just make everything up. Why not? That's what some of us are best at.