Why can't i put music from my computer on my iphone?

I have a pc and an iphone and I used to be able to take music from my computer and drag it on to my phone but I forgot how to do it and I can't figure it out again. Anyone know how to do it?

You need itunes then you go to your music documents then when you see the song(s) you want you right click it then click open with itunes then you sync the song with your iphone

ITunes?