How can I become a IPhone Tester?

By purchasing an annual iOS developer membership from Apple at developer.apple.com. Cost is $99.00 per year.

Huh? You mean, you want to receive prototype devices before they're released to the general public for sale in exchange for filing bug reports and so forth?

You don't. Apple does all this testing internally and through very limited use of firms which specialize in that sort of thing. iPhones don't go through a public beta test like a new patch of World of Warcraft; the public is not involved.