There are multiple ways of sucking at writing. You can be bad at putting a plot together, bad at making your words flow, bad at character development. But before you can be bad at all those thing, you need to learn the most basic building blocks of writing. People won't even get far enough into your story to decide if you're good at any of that if you don't have basic writing skills. Thus: Grammar Girl.
Not sure if it's "effect" or "affect"? I'm never sure. So I check Grammar Girl. And you should too. If you get comments about your grammar or you're just not sure that what you're doing is right, the best bet is to check. You can go ahead and bookmark this website, because unless you're an English teacher, you probably don't know EVERY rule.
Rating: 5/5 (you can't write a great novel without great grammar y'all)
Homework: proof read your work!
Check it out: http://www.quickanddirtytips.com/grammar-girl