Are you asking how to change the boot priority? Or do you know how, but it isn't working like it should? Generally, to change boot priority, there is a tab in the BIOS that lists all devices, and you move them up and down with '+' and '-' (or possibly some other keys)
Would you happen to know if your "BIOS" is UEFI or legacy BIOS? If it is legacy BIOS, then if the BIOS can see the hard drive it should boot from it as long as there is an OS properly installed on the hard drive, and no OS is installed on a higher priority device, like a CD-ROM drive. With UEFI, it can be a little more complicated.
If you are using Windows, and there is actually something wrong with the installation/configuration of your OS on the hard drive, one possible fix is to insert the Windows installation disk and run the repair option. However, this won't fix any problems that are actually in the BIOS.
To narrow down the problem, could you give an exact description of what happens when you try to boot your computer.