If you are wanting all the music from iTunes to be loaded onto your iPhone the above is great... However if you want to decide what music is synced and what music is not, you will need to change a setting
Check to make sure that the "Manually manage music and videos" option is turned on:
Connect your device to your computer.
Open iTunes. Make sure that you have the latest version.
Locate and click on your device.
Click the Summary tab.
Click the box next to "Manually manage music and videos."
Then you can drag and drop music from iTunes into the music of your iPhone.