An "s" on the end of a word can do two things: make a noun plural or possessive. A plural indicates there is more than one thing (apple/apples). A possessive indicates ownership, that something belongs to someone (Jane's dog). The apostrophe (') makes it a possessive, not a plural. This may seem basic, but people get their plurals and possessives confused all the time.

It's and its drives everyone crazy because it's seems to be the exception to the possessive rule. If Mary's dress means the dress belongs to Mary and John's car means the car belongs to John, then shouldn't it's toy mean the toy belongs to it? Well, it doesn't. It's stands for the contraction it is.
It's (with an apostrophe) means IT IS.
Its (with no apostrophe) means BELONGS TO.

Tip: Every time you see the apostrophe, say it is out loud to yourself and you will always get this right.

Here's another helpful way to think about this problem: his, hers, theirs and its are all possessives with no apostrophe. In that way its doesn't really break the possessive rule, it follows it.

Apostrophes can be confusing because they do more than one thing. They indicate possession (see above) but they also indicate contractions, like isn't, you'll, wouldn't. In contractions, the apostrophe stands for the missing letters (you will becomes you'll, leaving out the w and the i).

There are three theirs, and they're:
There: Place: Put it there.
Their: Possession: Their house.
They're: A contraction that means they are.

Your means something belongs to you: your glasses.
You're means you are.

People always confuse these two. They write, Your the best!
But that's not correct. To mean you are, you're must have the apostrophe.

Two is a number
Too means also
To is the preposition that indicates transfer or direction as in "give it to Jim", or "I'm going to the store."