There are a lot of ways to become a game tester. going to school to become a programmer , BETA test games online, or just get lucky. I would suggest going to school as the best and simplest way to become a tester.
Game developers love detailed reports from testers. Video game testers should have good documentation skills and be able to give detailed opinions of the games. A good way to get started testing games is to join the Beta program on steam. Go to File->Settings and click Change under Beta Participation.