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The first thing you need to know about MTG is the five colors. Those colors are Green, White, Blue, Black, and Red. Each color is associated with certain elements and other things. For example Blue is associated with wind and water while red is associated with fire, lightning, and sometimes earth. And Green represents life while Black represents death. It is important to understand the nature of each color so you can better understand how to build your deck.
Each color is has a land. Lands are the most important cards in a deck as you have to use them to cast spells. There are five basic lands (one for each color). Those five are Forest, Plane, Island, Swamp, and Mountain. Each land produces a colored mana for what color it is associated with.
Forest = Green
Plains = White
Island = Blue
Swamp = Black
Mountain = Red