Come on EBW, this is grade 4 shit. GrammAr not grammer.

Apostrophes can be used to shorten words or bring two together. This symbol: ' must
always replace a letter in order to be used properly!

You're: Short form of 'you are'. "You're paying too much for that
Prada bag"
Your: Representing something that belongs to someone else in the 2nd person.
Your hat doesn't match your shoes you skank!

They're: Short form of 'they are'. "They're bad spellers."
There: Demonstrating a location. "The dictionary is over there"
Their: Belonging to 'them'. "Their dictionary has never been
used."

Then: Demonstrating situational time. "I have to go to the mall then
go tanning."
Than: Describing an amount of something. "Obama is better than
McCain.

It's: Short form of 'it is'. "It's about time I got paid."
Its: Belonging to an object or common noun. "My Flight Attendant Manual is
green, its pages are full of useful information."

Andy's: Short for Andy is or something belonging to Andy. "Andy's a
fag." "Andy's car is hot and goes super fast."
Ross' When a name or noun finishes with an s you simply add an apostrophe. It
sounds like "Ross's" but that is incorrect, it's spelled Ross'.
Kids' Same concept. "Kids' wear."

Which: Describing a particular item. "Which one of you slutinas slept
with my boyfriend?
Witch: Noun, old fictitious hags with a broom back in the day. "You're ugly,
you witch."

Weather: Noun describing the outside conditions. The weather is great!
Whether: Conjunction used in indirect questions to introduce one alternative.
Whether or not you douche your sticky, illiterate vag is none of my business.

Another use of apostrophe just for good measure:
"Why the fuck's a Canadian writin' this?"
The i in is may be replaced by a ' and the g may be replaced by a '.

There's many more but I'll leave it at this. If you're unsure about something,
check on-line, there a hundreds of dictionaries and thesauruses, or you can just ask a
grown up like me!