How to put iTunes music on iPhone?

I know how to get music into my iTunes library but how to I get it in "on this iPhone" when I plug my phone in? ITunes is so confusing anymore. Lol
So basically when I sync my phone, all of the music from "'on this iPhone" gets put into my phone. So how can I put my iTunes library into it?
Sorry if it's confusing!

When you plug your iphone normally it should appear in the list. Then go to you libiry and take all the music you want and drag it to the icon of the iphone. It will syncronise and done;)