First of all, make sure you are wanting a games tester job for the right reasons. Many people think you just play games all day and get paid... in fact the work is long hours, very repetitive, and you'll need to very accurately describe scenarios, bugs and reactions in game, relay that information to the dev team, then retest and compare the results...
You'll need 3 things to be a games tester. Firstly a passion for games, secondly an analytical mind which can work through different permutations and test every solution to a problem, and thirdly the ability to communicate that effectively - that means you need to be a clear and accurate writer.
If you think you have those skills then work up a resume (work experience, games experience, any writing you have done either paid or free, any game dev yuio have done either paid or free)
Try and buildup some authority and experience, writing a games blog can be a great idea, writing articles and submitting them to games portals or magazines, in fact anything which proves your passion for games and your communication skills... then package all that up together and start sending it to the QA departments of games dev companies.
THere are a bunch of websites offering information and services for helping people become a games tester, some are better than others. Most of them offer little more than an overview, some hints on putting together a resume, and contact details of some of the software houses, but some also offer some more in depth information and resources...
http://www.betagamestester.com offers a free guide which should help you on your way.