I don't know if I've talked about this before, but if I did, it bears repeating! Many writers never learn the basics of writing. Their spelling and grammar are bad and they have no idea of how a story/article/book/whatever is supposed to be structured. If you want people to pay you for your writing, you have to give them good quality for their money. Do whatever it takes to get the writing basics: take a class, buy a book and study, whatever works for you. Nail your spelling and grammar, and your writing will automatically improve. Of course, you sometimes need to just write and let it flow...but if you have studied your grammar and spelling, they will be innate and you won't have to think about them while your creative juices flow. Until then, go ahead and write, but be prepared to go back through the work later and correct it.
My grammar is not always perfect, but I have several resources I refer to when I'm unsure of something. Of course, boo-boos will still slip through, but the majority of my work is clean, and that goes a long way.