Download Youtube Downloader
Then install it.
When you have installed it, go watch any video on youtube.
In the adress bar you should get something like this:
Copy that and open Youtube Downloader.
Select "Download a video from YouTube or other video site"
The adress should be written in a field.
Select Best aviable in the quality scrolldown menu.
Select the place where the song should be located when downloaded.
When done, select "Convert or play a video you have already downloaded"
Browse to the video
Select "MPEG Audio Layer (MP3)" from the scrolldown menu.
Click convert and wait.
Then you should have a file named