iTunes tells me that I have no storage space on my phone. It is the music that is taking up space. But I only have 145 songs on the phone that I have selected (3 playlists). The phone says that there are 600 songs. How do I get only the extra space back from the music I don't want on the phone.
On your iPhone, navigate to Settings > General > Usage.
At the top of this page there will be a list of apps in order of how much space they use. You can use this to decide what to delete to make more room for your music.
You can delete apps on the home screen by holding down on them until they start to wiggle. Hit the little "x" on any app to delete it.
You can delete music by sliding it to the left in your Music app. Then hit, "Delete."