So how does one learn to write, then, aside from simply practicing until you're good at it?
There are a lot of resources that weren't as readily available when I started college. If you're reading this, you have internet access, and therefore access to auditing online courses (link goes to MIT's courses), looking up information on whatever you need (link is snarky), and finding dozens of blogs devoted to something you need clarified. If that isn't enough, there are message boards devoted to many, many interests, forums where you can get feedback on your writing, and comment threads galore. The authors I feel most engaged with are the ones who blog or use Twitter (links go to outside blogs), and they're often transparent about their processes and how they learned their craft.
If you've done all that, though, and you can't figure out where you're going wrong, it's time to submit for a critique. That may be as basic as an alpha or beta reader, it may mean joining a writing group, or it may mean paying a professional to bleed red ink on your manuscript. (Be aware that you'll be paying that person to hurt your feelings. Choose wisely.) It can be uncomfortable, even painful, to have other people pick apart your writing, but growth often comes from discomfort and pain. As I mention here, you'll be motivated to change to keep from repeating such a painful experience.
The good news is, if all your feedback is about grammar or spelling, that's the easiest thing to fix. Your local library will have a style manual (outside link) or Strunk and White (Google books link), which are good places to start. Grammar is fairly straightforward, and there are rules you can learn. Once you've learned them, you have poetic license, and can do whatever you want with them.
I know I harp on grammar a lot on here, but it's not because it's the most important thing, or the hardest, or that I'm smarter than you (because I bet I'm not). It's because I'm a grammar prescriptivist with a decent background in writing mechanics. If you didn't learn these things in your public school years, you can still go back and learn them, and there's nothing to be embarrassed about. Or, you can pay someone to fix it for you, and my fellow grammar prescriptivist will be happy for the paycheck.
The second easiest thing to fix is word choice. There are words you use in business emails that you wouldn't use in Facebook posts, and vice versa. The same applies when you're setting a tone in what you're writing. If you're using a lot of sibilant syllables to describe what Handsome Love Interest says in Fair Heroine's ear, well, I hope it wasn't meant to be sexy, because she'll be wiping his spittle off the side of her face at her earliest convenience. Reading aloud can catch most language that sounds off, and giving your manuscript some space will usually take care of the rest. (I'll talk about that more next month, when I write a series of blog posts on editing.)
There are a lot more common issues that new writers run up against, but those two are the easiest to fix. I bring them up only to reassure you that, even though improvement sounds like a monumentally difficult task, once you're done learning a skill, you'll have a new tool at your disposal. Learning to write is a constant process, and you never stop. You just find a new skill to work on. You always know when that happens, because writing will feel more difficult again.