To play a game of Warhammer, you and your opponent will need to decide the size of battle to fight - the larger the game you want to play, the larger the total points values of your armies. The total points value determines the maximum points you can spend on your army.
For example, you may decide to play a 3,000-point game, in which case the total points value of all models in each players' army must come to 3,000 points or less. Normally, this will mean that the armies will actually be a shade less than 3,000 points, as it's quite hard to spend every last point.