One thing i cant understand that how to download mp3 songs on my iphone 6s

You sync songs to your phone through iTunes on your computer. See here: https://support.apple.com/en-us/ht201253

you can only be able to transfer the songs from yours laptop/imac/mac or from pc to yours phone usng iTunes only,but you can dowload songs from itunes app which is in yours iphone using credit/vissa card payment or from wallet but the songs which are downloaded from iTunes app is having higher clarity then yours transfered tones....