Please make a list of websites, dierections and include tutoirials. I am trying Avast right now and it keeps blocking Christian Academic websites and blocking amazon. Their certificates are out of date. lol You would think amazon would be ahead of the pack.
If your inquiring about Linux Operating Systems. There are plenty of reviews and tutorials on youtube. Linux Mint is often recommended for beginners.
First you need to figure out if your old computer is a 32 bit or a 64 bit machine. All my computers, including the oldest one I own, which is 15 years old, is a 64 bit machine.
First you download an ISO file. Then you convert that ISO file into a live bootable media. That could be a CD, DVD, Blu Ray, or USB Drive.
I looked these videos up on youtube. I suggest watching all of these in this order.
1) Watch This to learn what an ISO file is.
2) How to verify ISO checksums on Linux from a Windows computer.
3) How to burn an ISO to DVD
4) How to burn an ISO to USB Drive. He even shows how to boot into Linux Mint Live USB.
5) Download Linux Mint.
https://www.linuxmint.com/download.php
After you have created a bootable media, then you can try to booting into it. Linux Mint will boot you into a live session that won't change anything on your computer. The live media is for testing purposes, so that you can find out if the OS will run on your computer properly or not. When it comes to Linux, there are plenty of places online to get tech help. If your trying linux mint then you might want to join the linux mint forums.
http://forums.linuxmint.com There you can get help if something isn't working the way you hoped it would.
If you still feel that your ready to start using linux you got some options. You could just erase your computer and put it on by installing it or you could try something a little more safe. Remove the Windows drive and put in a new blank drive, and install Linux to that. Or you can even install Linux to a USB drive of some kind and use it for a while.