Who asked about Aposrophes? And I am not trying to be patronising at all, if I sound that way Im really sorry, I'm just trying to explain it as basically as possible to try and make it un confusing. I know a lot of people have trouble with them but it basically works as treydog said, although the way I remember it is;
Can this word become two words? (I'm going to steal treydog's examples 

)
It's raining outside.
It's has an apostrophe because it can become two words:
It is raining outside.
This works as a sentence, so you can feel good about correctly using an apostrophe here 

And now when it's (<--- lol it is 

) inccorect to use an apostrophe:
The scamp grabbed 
its head and screamed.
Can it be split into 2 words?
The scamp grabbed 
it is head and screamed
The basic rule of apostrophes is if the word can be split into two words then you put an apostrophe in there
Examples:
Can't = Can not (or cannot as one word)
Don't = Do not
Wouldn't = Would not
Shouldn't = Should not
Couldn't = Could not
And now the slightly more confusing part.
If you are talking about the posession of someone, the someone gets an apostrophe after their name.
Examples:
It was Charlie
's book. (Charlie is the someone and the book is his posession)
"Put Sarah
's glasses down!" (Sarah is the someone and the glasses are her posession)
This also works in other instances, for example:
It was all Charlie
's fault.
in this instance the word 'fault' acts like a posession. Basically when the name of someone is used you put an apostrphe between their name and the S. But there are exceptions to this rule as there always are in the english language.
I'll just mention one other thing that I really struggled with, I don't know about anyone else but I'll say it just in case someone finds it useful.
I could not work out the difference between 
effect and 
affect it's very difficult to explain the difference  but I'll have a go.
The word 
effect is used when describing the atmosphere of a situation;
As he entered the room the lightning struck a tree behind him, although he was not really one for drama, the 
effect really helped his 'bad guy' charade.
Mist floated about the mansion, adding to the spooky 
effectAn effect helps set the mood to something whether it's writing or a film or a play, but as I said before, if it is helping to set the mood then an effect is something that changes the atmosphere. If a director wants to show a character has depressed in a film he might have the colours of the film dulled down so they don't seem as bright. And a veiwer might say "I liked the visual 
effect the director used at that point in the film."
The word affect on the other hand is much more personal, you use it when saying how something will has an impact on a person.
"I think it 
affected her pretty well to be honest."
Affect = when talking about a person
Effect = when talking about something that adds to the mood of a situation.
I don't know if any of that was helpful to anyone, but I just thought I'd explain as simply as possible.