Watch how suits of armor are constructed in this How It’s Made program, originally aired on the Discovery channel.
Here is also an interesting article from the Scientific American on how medieval armor may have been more of a hindrance than a benefit on the battlefield: “Medieval Armor: Was it Worth the Weight?”
Though by the time combatants were wearing these heavy suits of armor, the knight had basically become obsolete with the increasing use of gunpowder-based weapons. As such, the knight decked out in his shining armor was more for show in tournament style displays than for actual, useful function on the battlefield.
*image is from Galerie des armes et armures, Metropolitan Museum of Art, New York, licensed under the Creative Commons Attriubtion-Share Alike license, uploaded by Mattes.