Also, never be drunk in front of a camera. Ever.
But curse words aside, learn your own goddamn language. Once upon a time you could get away with not knowing the difference between "their" and "they're" because in spoken conversation they sound the same. Online, everybody can see that you weren't paying attention in fourth grade when you mix up "your" and "you're."
Remember that text is going to be how you make your first impression over the internet; if every third word you type is misspelled, people will automatically assume that you're a moron. Even if the public school system fails you, there are browsers that come with spellcheck built-in. Me, I used Microsoft Word to type this article, simply because it checks my spelling as I type.
Also, keep texting shortcuts confined to your cell phone. Everybody understands when you use "2" instead of "too" because you were typing with your thumbs on a number pad. But you cannot let that habit sneak into your work or school emails or even message boards. And there's no reason for it, you have all the time in the world to construct and proofread your message. If you are so pressed for time that you can't spend the extra quarter of a second to type out the full word, you need to get up right away and finish your responsibilities. Don't even worry about closing the window - that would take almost a full second.