BIOS
BIOS, standing for Basic Input Output System, is the software that comes with the motherboard of your computer. It is really basic, handling the startup of the actual OS you have installed.
Technically, BIOS is the old version of the software, and the modern replacement is UEFI, but we call it BIOS anyway because we're quirky. UEFI stands for Unified Extensible Firmware Interface.
The BIOS settings page, which we also call BIOS (hey I didn't make the rules), is usually accessed by spamming some key while the computer is booting up.
Which key?
It depends from device to device. You can either look online for your particular computer, or you can try out all the likely keys. Every computer I've seen had the BIOS shortcut key as either ESC, F1 - F12, DEL or ENTER. Older computers also display it on the screen during the boot sequence but newer computers, especially SSD-equipped ones, boot up way too fast to display it.
Once in BIOS, you can change the Boot Order - this is the order in which BIOS scans all storage devices to find a bootable OS. Look through the settings to find this option, there shouldn't be many options.
For the process of dual booting, you will need to do this twice - once to ensure that USB has higher priority than internal storage, and once after installation (I'll tell you when) to ensure that within the internal storage, the Linux Bootloader (usually GRUB) has higher priority than the Windows Bootloader. Each time, make sure to actually save the settings.
Booting from the LiveUSB
The USB drive to which we wrote the ISO is now called a LiveUSB, because it contains a live version of the OS we can try out without installing.
Once you have given the USB boot device higher priority, you can plug in the LiveUSB and reboot. It should show your chosen OS ready for you to try out, with the basic apps installed, along with an option to install the OS. Try out the OS, make sure there are no black flags that disqualify it for your use, resolve red flags, find workarounds to yellow flags, and then you're good to install!
Filesystem Structures
In Windows, everything exists on the C:\ drive, with extra stuff on the D:\ drive if you have one.
Fun fact, A:\ and B:\ are names reserved for floppy disks in Windows convention, and any changes to it now will just break a lot of apps.
All program files live in the C:\ drive, unless installed elsewhere. User data is stored in a dedicated folder - C:\Users\ - with each user getting their own folder within it. Each of those folders contains all your favourite locations - Documents, Downloads, Pictures, all the shortcuts you see in the File Explorer.
Linux does something very similar. Everything exists under "root", represented by /. User data is stored in /home, with each user getting their own folder within it, and each of those folders containing all those same locations.
Installation
The one thing you need to watch out for is the location of the install. It will ask you if you want to remove Windows or install alongside Windows or a third manual option. The "Install alongside Windows" is usually a safe option and you can select the 150 GB free space we created earlier.
A slightly better option is to go for the manual option, and create two partitions. One will be 50 GB in size with its mount point as the filesystem root, represented by a single slash /. The second will take up the rest of the space, with its mount point as /home.
After this point, the process varies, but it is also relatively straightforward. You will want to do a network install, where it downloads any updates needed.
After installation, when it prompts a reboot, you will need to set the Linux Bootloader higher than the Windows one.