How to transfer music from computer to iPhone?

if your iphone is 4, you can download an app caaled itube and its free if you get the app and have wifi you enter the app and you will search the song you liked and you push on thesong icon and whn you do in the top right corner there willbe an icon bouncing you will push that icon and you will chuse the vid quality press doneand on the bottom a button is called chace i think. you wil push that wait for it to download then you push on playlists put create playlist put one kind of songs there.ex: country songs. you will then push add then it wil take you back. you will push cache again and the song should be downloaded you will go back yo playlists click on playlist you made push edit then the addition sign then it show you downloaded songs push the song then select. that is all. repeat steps if you need to add new song