Mac OS X (the operating system on a Mac) comes with some Apple applications, such as Safari to browse the Web, Mail to, well, receive and send email, iCal as calendar, etc...
One of those appllications is called TextEdit, and you'll find it in your Applications folder (which should be sitting in the rightmost part of the Dock, at the bottom of your screen). It's a very basic text processor/editor, but it may fit your needs.
You can also try a variant based on it, called Bean, which offers a whole slew of features TextEdit doesn't have. If you want something more refined and are willing to pay the price, you can opt for Microsoft Office for Mac, or a freeware such as OpenOffice, LibreOffice or NeoOffice (all three are cousins).
You can also try the latest version of Apple's iWork suite, which you might just adopt after a while. It's quite good.
You'll find all those apps and software via your favorite search engine...