Music that isn't purchased from the iTunes Store must first be imported into your iTunes program, and then synchronised with your iPhone in order to add it to the iPhone's music library. Audio files in MP3 format are compatible with the iPhone, and don't require converting before syncing them to the portable device. Once added to your music library, you can either choose to have the iPhone sync with your entire iTunes music library or choose to sync the specific tracks to the iPhone.
Open the iTunes program on your PC or Macintosh computer by clicking the iTunes icon on your desktop.
Select "File" on the iTunes menu and click "Add to library" on your Mac, or "Add folder to library" on your PC.
Double-click the folder that contains your MP3s to add it to the iTunes library. Plug the iPhone cable into the bottom of the iPhone and your computer's available USB port.
Click the iPhone tab in the iTunes source list, and select the "Music" tab next to the "Summary" tab in the main section of iTunes.
Select the check box next to "Sync Music," and click the circle next to "Entire music library." Click the "Apply" button to save your new synchronisation settings and sync the MP3s to your iPhone.
Open iTunes, and click "File" at the top of the iTunes menu.
Click "Add File to library" and double-click the MP3s to import them to the iTunes library. Plug the iPhone cable into the bottom of the iPhone and your computer's empty USB port.
Click the iPhone tab in the iTunes source list, and click the "Music" tab in the main iTunes section. Check the box next to "Sync Music" and click the circle next to "Selected playlists, artists and genres."
Scroll through the list of names in the Artists section and check the box next to the artist(s) of the recently imported MP3s.
Click the "Apply" button in the lower-right corner of the screen to sync the MP3s to your iPhone. Select the "Eject" button next to the iPhone tab in the iTunes source list to safely disconnect the iPhone from your computer.