I've met a lot of people who want to become writers, but who feel like they've already tripped out of the starting gate because of one big problem: they don't know their grammar. They can't tell apart a clause from a phrase, they don't have the first clue what a comma splice is, and they can't understand why editors just won't take them seriously. Most schools these days just aren't teaching grammar, and it's leading to a lot of people lacking basic English construction skills. So what's a budding writer to do?
I'm an experienced grammar tutor, and I can tell you with confidence that it's never too late to learn. The rules of grammar are all pretty simple once you've learned a rule and seen a couple of examples; it just takes practice and the desire to learn. If you don't know grammar but you want to write fluently and professionally, check out any of these resources to pick up the basics.
Writing a Research Paper, by Jonathan Ceely, Helen W. Dunn, Mary Tyler Knowles and Judith Robbins
The Elements of Style, by Strunk and White
Rules for Writers, by Diana Hacker
There are tons of resources for learning grammar online as well. It would be a big mistake to push grammar away as unimportant, the way I've seen many writers do. They tell themselves that James Joyce didn't write grammatically, so they don't have to either. The fact remains that Joyce learned the rules, and then broke them in very calculated ways. Being casually ungrammatical is a fast way to tell your reader that you don't care much about what you write. So if you're a little uncertain about dangling modifiers or misplaced modifiers or problems with parallelism, maybe it's time to brush up!