The internet is a wonderful thing. It's been an invaluable tool in bringing the world closer, helping one culture understand another, and, for me, anyway, making it possible to watch soccer from around the world pretty much any time I want.
But the internet has also had a deleterious effect on grammar and writing. Convenience has taken the place of accuracy and precision, and finding an online guide to grammar and writing can help recall the fundamentals that were drilled into you as a child in grade school.
There is not one particular guide to grammar and writing that is recommended above all others, but it's worth noting that none of these guides will tell you it's advisable to use "chatspeak." "Chatspeak" is using abbreviations like "u r" for "You are", or IMHO for "in my humble opinion." Chatspeak is where a good guide to grammar and writing goes to die.
It's funny bringing up the "you are" example in particular, since that particular phrase has suffered long before the internet ever came into existence. For whatever reason, most people can't grasp the difference between "your" and "you're". Yes, they're pronounced the same way, but they mean different things. "You're" is a conjunction meaning, literally, "you are." "Your" is modifier that indicates possession. I'll write a sentence - your own little guide to grammar and writing right here in this article - using each correctly.
You're right, a guide to grammar and writing would be useful.
Your guide to grammar and writing is very useful.
See the difference there?
Oooh... "there". That's another good one. People seem to have difficulty distinguishing between three words: "There", "Their" and "They're." I'll use them each correctly.
There is a goood guide to grammar and writing on the internet.
Their guide to grammar and writing is faulty.
They're going to need a guide to grammar and writing if they ever want to be taken seriously in the real world.
Each of the previous three sentences means something different than the other two. Memorize those sentence structures and how those three words are used, and you'll be halfway there in finding your perfect guide to grammar and writing.
I'd also encourage you to go to the library and find books on grammar and writing, and educate yourself on the proper use of punctuation (everyone struggles with apostrophes - there's no shame in that!) as well as tenses. Once you've done that, proper grammar and writing will become second nature to you.