How to download Mp3 music from Youtube to your computer

Hello! I was watching a music video on Youtube and saw a good song. I want to download it using Mp3 to my computer but I don't know how to do it.
If you know how to download Mp3 music from Youtube to your computer for free and easy to create sonnerie samsung can guide me. I have tried the methods available on Google but with no success. Thank you for reading the article!!!

Easy, write "ss" in the url before of "youtube"

Example: www.youtube.com/forobeta
Download: www.ssyoutube.com/forobeta