Holding down The Option key (and keeping it depressed) while turning the MBP on will give you the boot device selection screen, where you can choose the OS or device you want to boot.
If you want to make Windows the default boot option, then boot normally to OS X, go to System Preferences -> Startup Disk.
The selection can also be made in Windows with the support software.
This information, and anything else you might need to know is documented both in the Bootcamp manual:
http://manuals.info.apple.com/en_US/boot_camp_install-setup_10.8.pdf and in this Wikipedia article:
http://en.wikipedia.org/wiki/Boot_Camp_(software)
Searching this forum, Apple support site or Google will also answer a lot of questions.