The goal
Each player starts with a life total — 20 in most formats, 40 in Commander — and you win by knocking your opponents down to 0. You do this mainly by attacking with creatures, backed up by spells that clear the way.
Mana: how you pay for things
Every card costs mana to play. Mana comes from lands — you may play one land from your hand each turn, and you tap (turn sideways) your lands to produce mana. Tapped lands untap at the start of your next turn, so your mana refills every turn. The longer the game goes, the bigger the spells you can cast.
WhiteOrder, healing, armies of small creatures
BlueCard draw, counterspells, trickery
BlackDestruction, sacrifice, power at a price
RedDirect damage, speed, aggression
GreenBig creatures, ramp, raw growth
The card types
Your mana source. Free to play, one per turn.
Attacks and blocks. Has power (damage it deals) and toughness (damage it survives), shown bottom-right like 3/3.
A one-off spell you can only cast on your own turn. Does its thing, then goes to the graveyard.
Like a sorcery, but castable at any time — even during an opponent's turn or in response to their spells.
A lasting effect that stays on the battlefield.
A lasting object — equipment, trinkets, machines. Usually castable with any colour of mana.
A powerful ally with abilities you can activate once per turn. Opponents can attack it.
A rarer type you attack to break for a reward. Don't worry about these yet.
How a turn works
Untap & draw
Untap all your cards, then draw a card. (The player who goes first skips the draw on turn one.)
Main phase
Play a land, and cast creatures and spells by tapping lands to pay their costs. Creatures can't attack the turn they arrive — they have "summoning sickness" until your next turn.
Combat
Choose which of your untapped creatures attack, and who they attack. The defender chooses blockers. Blocked creatures deal damage to each other; unblocked attackers hit the player's life total.
A creature dies if it takes damage equal to or greater than its toughness — otherwise damage wears off at end of turn.
Second main phase & end
Cast anything else you can afford, then pass the turn. If you have more than 7 cards in hand at the end of your turn, discard down to 7.
A few things worth knowing
When you cast a spell, opponents get a chance to respond before it happens. Responses resolve last-in, first-out.
If you must draw from an empty deck, you lose. It rarely comes up.
The most popular casual format: 100-card decks, 40 life, and a legendary "commander" creature you can always recast from its own zone.
The golden rule — cards explain what they do. If the card text and these basics disagree, the card wins.