I would like to know how to convert Youtube files to MP3 on my Iphone 4?

I have an iPhone 4 & would like to know the simplest way to conver youtube songs to MP3 on my device with out having to do any sort of "jail breaking" as I do NOT want to do that. It would be a big help if I can do it on my iPad as well as I have iCloud.
If anybody can give specific instructions it would be very helpful.
I'm relatively new to IOs.

Google ''convert youtube video to mp3'' and loads of sites will come up. When you've converted the file you want, make sure you save it to your computer. Then on your itunes go to file and click add file to library. Find the file in your computer and then sync it to your iphone. No jailbreaking or anything. Answer mine?