Refer this article to add our music to iTunes library:
iTunes: About the Add to Library, Import, and Convert functions
After music appear in iTunes library, you can connect your device to your computer an sync.
Refer to this article on syncing:
iTunes: Syncing media content to iOS devices and iPod
Note: make sure you have signed in iTunes on your PC with your AppleID, and authorize the computer.
If you have your other music in a file, and if it is a proper format,(and mp3 works just fine) you just need to add the music, where ever it is, to the i tunes library. At the top of i tunes, select add to library, then tell i tunes where it is. It will now show up so that you can select it for syncing.