I have itunes on my computer, but it is not recognizing my new iphone to add as an authorized device.

We are new to the iphone (5c is our version) and thought we could just plug it into the computer and our itunes would recognize it so we could put music on the phone. I was able to download purchased music from the cloud to my phone, but since my husband's phone is under another Apple ID, it won't let us download purchased music from the cloud. How do I get my 2 iphones to be authorized devices for my itunes library. Other than itunes, we are completely new to the iphone/MAC experience. Any help is appreciated.