For becoming a Mobile Application Tester you will have to join a company which is working in mobile domain.
Testing of mobile Application is a bit different than conventional software testing of web applications and desktop application.You need to have domain knowledge of mobile platforms,how to install application on these devices ,how to test the mobile applications and so on.I am having my Blog http://mobileapplicationtesting.wordpress.com/ where I keep updating on Mobile Application Testing.
If you have to expertise in this domain then you will have to take step by step
1.First know about mobile OPERATING SYSTEMS
2.How to install applications on various devices.
3.Actual testing of any application on device.
and so on there are many steps ahead...