iTunes library

We'll assume that you mean an iPhone, and not a non-Apple one. (Basic, I know, but not everyone appears to appreciate that iTunes is only designed to work with Apple products.)

Each device has its own iTunes settings, so you need to set up some options, to add music to the phone. You only need to do this once, then a Sync will do all the work for you each time you plug the phone into the computer. (There is an option to Sync over Wi-Fi, but you can investigate that at a later time.)

• plug the iPhone into the computer
• click on the device icon once it appears at the top of the iTunes window:

• iTunes will the show you the Device/Settings/Summary pane
• click on Device/Settings/Music (1.):

• click in the Sync Music (2.) box to turn on the tick
• choose either (3.)
  • Entire music library - which will add everything in your iTunes Music Library to the phone. You will not see any of the sections numbered 4
  • Selected playlists, artists, albums and genres - in which case option 4 opens and you choose which items you want on the phone
• click on the Apply (or Sync) button that is at the bottom of the iTunes window (not shown in screenshot)
• Once the Sync has completed, safely disconnect the iPhone by clicking on the eject icon that will replace the spinning arrows (just above the battery icon in the picture above)

At subsequent connections between the phone and iTunes, a Sync will automatically take place and:

• • if you're syncing the entire library, any changes (additions to, removals or deletions from, and changes to Playlists) in the Library will be made to the phone (synchronised)
  • if you're syncing selected playlists, artists, albums and genres, changes to those selected items will be made to the phone as well, while other changes will not affect the phone