I'm totally new to Linux
so please forgive my lake of knowledge!

Okay, #1- First I tried to install Linux Mint -which is Ubuntu based distro!- on an old "IBM S50 ThinkCenter" desktop!
& I managed to burn a CD after failing to boot from a live USB to find only the same message again & again (1962: No operating system found. Press F1 to repeat boot sequence.)- after many tries I gave the cd a shot but this time surprisingly the Mint logo shows-up but my Pc doesn't like happy endings! so after I open the "Install linux" icon & choose "English" as a language the "Preparing to install Linux" screen freezes! So I gave up on that PC.

#2- Now I want to install Linux on my laptop -"Toshiba L650"- this time I try another Ubuntu based distro "Elementary OS" to find a new error this time!: after the glowing loading logo of the Elementary splash a black screen showed-up saying:

[ 0.317932] platform wdat_wdt: failed to claim resource 4
[ 0.317993] ACPI: watchdog: Device creation failed: -16

BusyBox v1.22.1 (Ubuntu 1:1.22.0-15ubuntu1) built-in shell (ash)
Enter 'help' for a list of built-in commands.

(initramfs) Unable to find a medium containing a live file system
_

So, I'm determined to install Linux & learn command line too within a month!
Any suggestions or info would be greatly appreciated
Have an awesome day

Some old computers won't boot from usb drives; my old Fujitsu-Siemens won't. So you are right to go down the CDROM route. But I wouldn't recommend Ubuntu for this kind of hardware. Ubuntu is a big modern Linux distribution that requires big modern computers to run on. The same for Mint, the other distro usually recommended to newbies.

I suggest you go for AntiX. It's a distro designed for "antiques". You won't be able to get the full version on a CD but I think the "base" version will fit and you can add extra programs later. Check the md5sum after downloading it and burn it at the slowest rate possible. Some of the errors you've been getting could be due to disc corruption.

When you install, use the command line installer, not the graphical one. Command line installation is always more reliable but not all distros support it. I don't think Mint does. Here is a link to some instructions for doing it with AntiX.