How do you get gamerscore on Xbox 360?

In order to unlock Gamer-score points you must play Xbox 360 games and complete a mission, or do something special that will unlock at the bottom of your screen. Each Achievement is worth a prescribed amount of points. To view your point balance or what Achievements your game has to offer, press the center Xbox 360 button on your controller, go the the left until you see "Achievements" and select your game.