The BIOS "should" automatically detect if there are 2 or more operating systems and automatically give you the options of what to boot from when you turn it on. This should work the same whether its multiple hard drives or multiple partitions on a single drive (it should also boot a specific one of those operating systems automatically after 30 seconds, usually the sata port with the lowest number assigned to it). If your specific motherboard doesn't do this automatically (it should but I'm not 100%) then you would need to make some changes on your motherboard and that step will be unique to your specific BIOS. If it does not do this automatically its likely the OS did not install correctly/corrupt/invalid etc etc.
Alternatively you...