How to install an OS after you build a PC?

From a USB Drive? After you get the PC all built, how do you install an OS?  Don't you go into the BIOS and do something? Thanks.

Do it from a bootable USB drive or DVD.You may have to go to the bios to make it boot from the DVD or USB drive, but it's easy. Your user manual for your motherboard will show you where the boot priority is and how to change it. You'll need to have either ready before you start, so plan ahead.

If you're installing windows and have an optical drive, you're set. Just put the disc in your drive and power it up. 

First make a usb stick bootable on a diffrent computer using disk part. second copy all installation files from the windows install disk or iso, to the stick. Third go into bios, and set you boot device priority usb drive as first boot. done.