First off, let's begin our tour with Wikipedia's entry on the Middle Ages. http://en.wikipedia.org/wiki/Middle_Ages (or, if you don't mind abstruse language, try the Columbia Encyclopedia http://en.wikipedia.org/wiki/Middle_Ages ).
Now that you've got a general understanding of the era, click on over to http://www.eyewitnesstohistory.com/mefrm.htm and explore what the people of the time had to say. Still can't get enough? Try http://www.fordham.edu/halsall/sbook1n.html#Anglo-Saxon%20Britain
Let's narrow our focus here. Have a look at Wikipedia's article, "Knights." http://en.wikipedia.org/wiki/Knight Explore the links a little.
Done? Check out the websites below. And don't forget to visit your local public library, the end-all be-all of reliable information!
http://www.medievalknights.com/ - general website
http://www.deremilitari.org/resources/resources1.htm - Medieval Military History
http://www.heraldica.org/topics/orders/ordhist.htm#intro - History of the Orders of Chivalry
http://www.themiddleages.net/ - general website
Novels set in the Middle Ages. I suggest you flip through a few to get an idea of what the language is like, and *gasp*, maybe even read a few all the way through. Ivanhoe is the most well-known, and it never hurts to read good literature. Don't worry, this is all legal. These books are all public domain.
Sir Walter Scott's Ivanhoe - http://etext.library.adelaide.edu.au/s/scott/walter/ivanhoe/
More Sir Walter Scott - http://www.online-literature.com/walter_scott/
Novels set in the Middle Ages - http://onlinebooks.library.upenn.edu/webbin/book/browse?type=lcsubc&key=Middle%20Ages%20%2d%2d%20Fiction
For the books on this list, you'll have to visit the library. They're relatively recent and easy to read, but Sir Walter Scott is always the best (see above)! http://www.uoregon.edu/~midages/novels.shtml
Read all about the major figures of Arthurian legend in in-depth detail http://www.lib.rochester.edu/camelot/mainmenu.htm
Note: The Middle Ages covers quite a lot of chronological ground, and unless you elaborate on the era you're looking at (The Crusades, the Norman Conquest, etc.), I can't do more.
Happy writing (or should I say reading?)!