THEN is most often used as an adverb that indicates time or order.
I’ll finish the laundry then start cooking dinner.
She ran through all the ice cream, then she turned her attention to the chocolate chip cookies.
THAN is most often used as a conjunction used to indicate comparison:
She is prettier than I am, but I am a much nicer person.
My brain can think faster than my fingers can type.
TWO is a number:
I have two papers due on Friday.
TO is often used as an adverb indicating direction toward something:
Go to the post office and pick up your package.
TOO is generally used to indicate inclusion or extremity:
This chocolate sundae is too good.
If you go to the movies, call me; I’d like to go, too.
Finally, apostrophes are only used to indicate possession or to stand in for the missing letters in contractions – never use them to make plurals or to change the tense of verbs:
That is Susan’s kite and I can’t stand it when you do that are correct.
Jessica want’s to go to the party and she has a huge collection of stuffed animal’s are not.
There’s some academic (linguistic?) debate about whether or not to put apostrophes with letters, abreviations or acronyms to make them plural (“We check ID’s“), but I DON’T. For me, the difference between the upper cases of the abreviations and the lower cases of the s that makes them plural is enough of a separation, so I write T.V.s, DVDs, and IDs, but I know that a lot of folks put the apostrophe in there and aren’t considered wrong.