how can i sync my music

If you don't have a subscription to Apple Music you can connect your iPhone to your computer with a cable and either use Finder or iTunes to sync content across from computer to phone. You can't sync content from your phone to your computer.

Sync iPhone with your computer – Apple Support (AU)

On your iPhone, iPad, or iPod touch

1. Go to Settings > Music.
2. Turn on Sync Library. If you don't subscribe to Apple Music or iTunes Match, you won’t see an option to turn on Sync Library.