How do I fight bosses on terraria?

Well first you gotta summon them. To start off I'd kill the easiest boss the eye of Cthulhu you need a demon alter and 6 lenses to make an item that summons him. (You use the demon alter like you would with a work table or what ever there called. After you made the item you need to summon him go outside and summon him but make sure it's night for tips of killing him read the article. http://terraria.wikia.com/wiki/Eye_of_Cthulhu