I have the latest iTunes on a brand new laptop. When I drag a song from my library over to my iPhone it will not let me put the song on it.

Under sync music my only choices are "entire music library" or "selected playlists, artists, albums, and genres". I have that second option checked, but the whole area is gray. To un-gray it I need to click the box that says "Sync Music". When I hit the button to sync music though I get a message saying that all existing songs and playlists on my phone will be replaced with songs and playlists from my library. I don't want that. I want to keep what I have on my phone, and then add single songs here and there. Please help!