How to Add Music to your iPhone with iTunes
Step 1: Launch iTunes and plug in your iPhone.
Step 2: Create a new "playlist" in iTunes. Name it whatever you want–say, "My iPhone Playlist" or something like that.
Step 3: Drag the songs you want from your iTunes library to your new playlist–in this case, "My iPhone Playlist"
Step 4: Click on your iPhone and go to the Music tab. "Sync Music" should already be checked. Now click on the button "Selected playlists", then tick off "My iPhone Playlist" (or whatever your playlist is named).
Step 5. Sync your iPhone and when it’s done, the songs from "My iPhone Playlist" should now be on your iPhone. If you want to make any changes to your mp3 library on your iPhone, you’ll have to modify the songs within your playlist, then re-sync again. Yes, I know it can be a pain.