Too many people misunderstand the proper use of apostrophes. The worst is "it's" versus "its." If you simply remember that "it's" is always a contraction (usually for "it is") and "its" is always for possession ("the flower lost its petal"), you'll be fine. For more information on how to use the apostrophe properly, learn from Bob the Angry Flower and The Apostrophe Protection Society.
Another way to remember it is that "its" compares to "his" and "hers"—although I have seen "her's" which is never correct in any context.
When you end a sentence as an exclamation, one exclamation point ("!") is sufficient to convey the idea of strong feeling or emphasis. Three is sometimes used for extreme stress—no more than that—and it's considered unnecessary and tacky. You never end an exclaimed question with "?!"/"!?". Only an exclamation point is used, and you never use more than one question mark in any circumstance.
When you omit part of a thought the triple-dot or "ellipsis" ("...") is used, and four are always used at the end of a sentence—never more than that, never less. When you end a sentence with an acronym or an abbreviation you never end up with two successive periods, one is dropped ("etc." not "etc..", "T.L.A." or "TLA." not "T.L.A..")
Punctuation always goes inside quotes. All of these are incorrect:
"This is wrong".
He asked "is this right?".
He said "I guess not", and shrugged.
Although I will sometimes deliberately break this rule when I feel clarity is necessary, as seen above.
These mistakes are especially prevalent online.
"A lot" is two words. "Alot" doesn't exist in any lexicon, because it's wrong. I had an english teacher in junior high that would fail any paper that used "alot" since she stressed this point at the beginning of the school year and had two signs in her classroom repeating this fact. (She did allow you to fix the paper and re-submit it, but a re-submitted paper could never get a 100% or "A" grade.)
"There" is a location ("it's over there"), "their" is plural possession ("it's their house"), and "they're" is plural contraction ("they are").