Hold down the OPTION/Alt key shortly after the startup chime sounds and then select Mac OS X from the bootable volumes selection screen.
If you installed all the proper Boot Camp Windows drivers, there should also be a small icon in the Task Bar in Windows, where you should be able to boot into Mac OS X.
To set Mac OS X as standard boot option, go to Mac OS X > System Preferences > Startup Disk > select Mac OS X partition.