Basic Deck-Building guide

There are 5 colors/elements:
red, black, blue, white, green
Mountains, Swamps, Islands, Plains, Forests

These are shown on the back of every card. Adjacent colors are always "allied." Opposite colors are enemies or counters.
Blue+white, green+red, blue+black are all good combos, but hardly anyone will play black+white. Blue often counters green effectively.

When building a deck, you usually focus on one or two colors. Three-color decks are hard to build because the number of lands in the deck is limited. The more colors you have in your deck, the harder it is to draw the right lands for the colored mana. Often, three-color decks use tricks, such as creatures that can produce mana or spells that bring lands directly from the library into play. Five-color decks are ultra rare and barely playable.

Deck building isn't ultra complicated. You should aim for 60 cards. Approximately 24 creatures, 12 spells, and 24 lands. The only rule is that each regular card can be in the deck at most 4 times. Lands are exempt from this. Creatures are usually included 2-4 times to increase the chances of drawing the desired ones. That's why 60 cards—otherwise, an imbalance occurs when drawing. Additionally, you should try to achieve a flat mana curve. The mana curve of the deck is determined by plotting the mana costs of all cards in the deck on a graph, i.e., the number of cards per cost. (Exceptions to the rule are all specialized decks, e.g., focusing on trample damage or many cheap Goblins).

To start, you should look at which cards you have and decide on a theme around a specific gameplay mechanic or special card.