Each player starts the game with 20 life. Knock your opponent down to 0 life, and you win. The most common way to do this is to summon creatures and attack with them.
Creature cards are the most important part of many Magic decks. They’re really easy to spot—just look at the lower right of a card. If you see a pair of numbers separated by a slash, you’ve got a creature card. Once it’s on the battlefield, a creature continues to attack and defend for you until your opponent can find a way to take it out.
Most games become a race to see who can deal the most damage first. Summoning the best creatures will help you win that race every time. Check out the section on casting creature spells.