How do I add the songs in my library to my iPhone?

Recently I have switched to a new computer and from the old computer, I saved all my MP3's into 1 file and into the hard drive, to which I put in the new computer. After re-installing iTunes and dragging all my MP3's into the library, I noticed my iPhone had old songs that I wish to delete, however I wish to delete ONLY my iPhone songs, while still keeping the new songs in my library. Is there anyway to do this? 1,000 thanks.