This is not really a Volumio issue, you will need to read the instructions you got with your PC as settings for booting from USB depends on BIOS supplier, PC brand and model. There are many thousands of possibilities.
Your documentation might show you which settings for your particular BIOS need to be set to boot from USB as a primary device.
If your BIOS does not allow you to set USB as a default boot option, then @Wheaten 's remark is your only option.
In case you don’t have any documentation, then you have to refer to your vendor, there is not much we can do unless we exactly know which PC you are referring to.
Read the following post and answer in that thread and perhaps the team or a community member can help.