Good question. The use of the apostrophe causes much debate and the word "it" adds some complexity. To answer your question first, "it's" is simply an abbreviation of "it is" while "its" implies...
You use have when your talking about yourself like "i have a computer" you use has when your talking about someone else like "she has a computer" simples:)
A page is in a book, worksheets or other stuff that are together. A word is a thing that you write in a paper, book or in a speech, without words we won't be able to write and talk.