It would really help if anybody can link a good guide on basics of nutrition, calories, metabolism and their effects on body, weight. I never cared about these things and I never understood all the calorie calculations on the diet plans or why they should be the way they are. I searched on internet, but these topics are so full of litter. I would appreciate if anyone can link a to genuine nutrition basics.

Edit: It's better if the guide is more general, than american-centric.