How do I stop my Mac from syncing my whole library to my iPhone?

Solution 1:

This is fairly straightforward to disable. Follow these steps:

• Launch iTunes
• Go to iTunes > Preferences
• Select the Devices tab
• At the bottom, uncheck the Prevent iPods, iPhones, and iPads from syncing automatically
• Exit Preferences

Let us know if this doesn't resolve your issue.

Solution 2:

Once you've disabled the auto-sync as per Monomeeth's answer, you can then choose which songs it will transfer in future, at manual sync.

• Connect your phone.
• Launch iTunes.
• Wait for the phone icon to become solid, then click it.
  
• Select Music on the left sidebar
• Select something other than Entire Music Library You can then choose which songs/playlists etc it should sync from lower down the page [not shown on photo as I have 'all' selected.]