Just in case anyone who doesn't know how to add music to iTunes library. Click "File" on iTunes menu bar. If the menu bar is closed, press "Ctrl"+"B" to open it. Then choose "Add File to Library" or "Add Folder to Library" to import music from your computer disk. Now you can follow the link above, or have a look at this article, which is also good. How to Transfer Music from Samsung to iPhone
iTunes can help you.
1. Plug your iPhone > Open iTunes.
2. Click on the iPhone logo under "Devices".
3. Click on the "Music" tab.
4. Choose "Entire Music Library" to put music from iTunes to your iPhone;
Choose "Selected Playlists, Artists, and Genres" to add certain artists or playlists to your iPhone.
Click on the "Sync" button.