There are several things that need to happen for your PC to boot from a USB drive. For many people (and computers), it will work automatically without any problems. But given that you're reading this, chances are that this is not the case. So here's what you need to do to make your PC boot from USB.
See also: How to install Windows 10
How to boot from USB: make your drive bootable
First, you need a USB flash drive which is bootable. It sounds obvious, but your computer won't be able to start unless the drive is formatted correctly and has the appropriate files and software which enable it to boot.
A common mistake is to copy the ISO file to a USB drive and assume this is all that's needed for it to boot and install Windows. In fact, you need a utility which can write the files to the drive and also make it bootable.
These tools should always warn you that they'll format your USB drive, so make sure you've removed and backed up any files you don't want to lose before starting. You'll also need to have downloaded whatever software it is you're planning to boot from, be it Windows, Linux or something else.
See also: How to install an SSD in your PC
How to boot from USB: select the boot drive
Many PCs and laptops are set to check for a USB drive first before trying to boot from a CD, DVD or the internal hard drive. But not all are configured this way. You might find that plugging in your flash drive before pressing your computer's power button does nothing: it just boots into Windows from the hard drive (or fails to boot if your Windows installation is damaged).
In this case, you need to look out for a message on the POST screen - the first thing you see on screen. It's usually at the bottom of the screen and says something like 'Press F11 for boot options'.
Some PCs and laptops don't even show this in order to boot up quicker, so you'll have to check your manufacturer's website to find out which key to press, or even your motherboard manual.
Alternatively, try to get into the BIOS as the computer starts up by pressing Del, F2 or whichever is the appropriate key for your computer. Again, look for a message on screen which tells you which key is for the BIOS, or search Google for the BIOS key for your particular make and model. (See also: How to get into your PC's BIOS)
Once in the BIOS, look for a menu or an option that lets you see and change the boot devices or boot priority. This will vary from BIOS to BIOS, but it's usually fairly obvious.
If you see something like the menu below, designed for older hardware, try each USB option in turn to see if they work.
A tiny minority of old PCs may lack the ability to boot from USB, but anything within the last decade should have the option.
Different BIOSes work in different ways, but use the information on screen to work out how to make a USB drive the first boot device.
See also: How to build your own PC.