Calm down, calm down. It's not that bad. Sure, having decent grammar is important to writing. And sure, the rules of English grammar can be hard to understand at times (and sometimes they even change!). But there are many ways you can improve your grammar without having to go back to high school English class.
One of the best resources on the web is quickanddirtytips.com's Grammar Girl. This site features a wealth of easily understandable information on thorny topics like affect vs. effect, who vs. whom, and correct apostrophe use. And she also has a book:
Believe it or not, the comic blog The Oatmeal can also be a good place to go for easy to understand grammar comics punctuated with some risqué humor. Try "How and why to use whom in a sentence" or "How to use a semicolon (the most feared punctuation on earth)."
For those who prefer to go old school for their grammar info, there's the classic reference book The Elements of Style. The title is somewhat misleading, because this book has a lot more to do with grammar and effective writing than personal style.
Do you have a go-to grammar reference, on the web or off?