what is the proper way to transfer video/music between my iphone and mac/pc?

i'm using the latest itunes. and i had some videos on my imac and i was trying to drag and drop from my imac to the itune folder, but it won't let me. it only allow me to do it thru itune stores. can someone give me steps by steps instructions how do you transfer music/video files between my iphone 5s between my imac/pc? thanks in advance.