By far the easiest way to install Debian GNU/Linux is from a set of official Debian installation images. You can buy a set of CDs/DVDs from a vendor (see the CD vendors page). You may also download the installation images from a Debian mirror and make your own set, if you have a fast network connection and a CD/DVD burner (see the Debian CD/DVD page and Debian CD FAQ for detailed instructions). If you have such optical installation media, and they are bootable on your machine, you can skip right to Chapter 5, Booting the Installation System. Much effort has been expended to ensure the most-used files are on the first CD and DVD image, so that a basic desktop installation can be done with only the first DVD or - to a limited extent - even with only the first CD image.
As CDs have a rather limited capacity by today's standards, not all graphical desktop environments are installable with only the first CD; for some desktop environments a CD installation requires either network connectivity during the installation to download the remaining files or additional CDs.
Also, keep in mind: if the installation media you are using don't contain some packages you need, you can always install those packages afterwards from your running new Debian system (after the installation has finished). If you need to know on which installation image to find a specific package, visit https://cdimage-search.debian.org/.
If your machine doesn't support booting from optical media, but you do have a set of CD/DVD, you can use an alternative strategy such as net boot, or manually loading the kernel from the disc to initially boot the system installer. The files you need for booting by another means are also on the disc; the Debian network archive and folder organization on the disc are identical. So when archive file paths are given below for particular files you need for booting, look for those files in the same directories and subdirectories on your installation media.
Once the installer is booted, it will be able to obtain all the other files it needs from the disc.
If you don't have an installation media set, then you will need to download the installer system files and place them on the a connected computer so they can be used to boot the installer.