Punctuation is something a lot of people usually miss off entirely. For a start, you always need to put a capital letter at the start of a sentance, a name, an expanded acronym (for example, expanding "UN" to "United Nations"), or a letter I on its own. This also applies to "I've", "I'm", and similar words where an I is on its own before an apostrophe.
Exclamation marks appear at the end of a sentance for emphasis, or to indicate that a character is speaking loudly or shouting. A single exclamation mark will do; there isn't any need to put more than one.
Question marks appear at the end of a sentance to indicate that someone is asking a question, or may sometimes be used to convey shock or disbelief (for example: "You did what?"). Again, a single mark will do as extra ones serve no real purpose.
Commas are used to indicate a pause in a sentance, (for example: "Wait a second, I need to talk to you.") or an item in a list (for example: "I have to buy milk, eggs, sugar, flour, and butter."). When using the latter form, put "and" before the final item (as shown in the example) to indicate that the list is coming to a close. If you have an extremely long sentance, break it up with commas to make it easier to read, especially if it needs to be read out loud.
Periods indicate the end of a sentance. Never forget to put them there, though
there are a few exceptions when quotation marks are thrown into the mix.
Three periods, one after another, (...) indicate a pause, or a trailing off (for example: "Wait... what's that?", "But I thought you said..."). They can also indicate two halves of a sentance, (for example: " 'Before you go...' he said, '...remember to lock the door on your way out.' ").
Quotation marks indicate speech, though either single (' ') or double (" ") quotes may be used. They are also used to indicate a direct quote, but if you quote in a sentance make sure not to use the same marks as you did before (for example: "Oh, they said, 'You had better be back before ten, or we're coming to find you'.").
Apostrophes are generally used to indicate possession, (for example:
"Marcus's") but it can sometimes be hard to determine where to put the apostrophe,
or whether to put it at all. They are also used in conjunctions (making two words
into one, such as "I've"). Just look at the following for reference:
Conjunction: It is = It's [Apostrophe]
Singular possessive: Belonging to it = its [No apostrophe]
Singular possessive + name: Belonging to Manny = Manny's [Apostrophe]
Singular possessive + name ending in S: Belonging to Marcus = Marcus's [Apostrophe]
Plural possessive: Belonging to the group = Group's [Apostrophe]
Plural possessive + name ending in S: Belonging to the Yoshies = Yoshies' [Apostrophe]
Colons (that's the two dots on top of each other, ":") are often used to indicate that either a list or example of something just said is coming up. (The "For example:" notes on the page are a good example.)
And, as a final note, supposing you could use a comma or a semicolon at the same point? You could use a comma to indicate a pause, but a semicolon would define an example... this brings us to semicolons (the dot over the comma, ";"). When you could use either one, just combine them. Simple, right?