If you don't need to off-load songs from the iPod, here's how to set it up to Sync Music using automatic syncing. It may be set up to load content manually or it may be set to sync with a different iTunes library (not the one you currently use).
Select iPod in iTunes. In the latest iTunes, click its device button on horizontal bar, to show its settings screen in iTunes window. Here's a screen shot of iTunes window (upper left corner) showing the iPod's device button (circled in red)

On the iPod's settings screen, along left side (in the sidebar), under Settings, click Music. The iPod's Music settings screen is shown to the right.
NOTE: Any songs on the iPod that are not in your iTunes library (on the computer) will be lost.
Checkmark the box for Sync Music. This turns ON automatic syncing, or (if needed) associates the iPod to your current iTunes library. If your iTunes music library can fit on the iPod, you can use the option to sync Entire music library. Otherwise, use the option to sync Selected playlists, artists, albums, and genres. Then make you selections from the lists below. For example, you can create and maintain (one or more) playlists in your iTunes library, with songs you want on the iPod. Back on the iPod's Music settings screen, under Playlists, find and select (checkmark) those playlists. Click Apply to sync your music library (or selected portion of it) to the iPod. Going forward, iTunes automatically keeps the iPod "in sync" with changes to your music library (or selected portion of it), when you connect iPod (or click Sync if already connected).
If that's not the problem, please post back with any more details.