Begin by not typing in all capitals. That's considered to be shouting, and it's rude.
- Connect your phone to the computer
- click on the device icon that will appear at the top of the iTunes window

- that will display the Device/Settings/Summary pane
- click on Device/Settings/Music (1.) as shown in the screenshot below

- Click the Sync Music (2.) box to turn on the tick (check mark)
- Choose either (3.)
- Entire music library - which will add all the music in your iTunes Library to the phone (note that the boxes in 4 will not be displayed)
- Selected playlists, artists, albums and genres - this will then display the sections in 4, and you can choose the items you want to add to the phone
- because you have changed your selections, click on the Sync (or Apply) button in the lower right corner of the iTunes window (not shown in screenshot)
Future connections between the phone and the computer will automatically use the settings that you made previously.