Connect your iPod to your PC and run iTunes. Select your iPod in the source list in iTunes. In the main iTunes Window (under the "Summary" tab) scroll down to the bottom. Uncheck "Manually manage music and videos".
Setting iTunes like this ensures that your music and videos are automatically synced with iTunes. You can check this by clicking on the Music Tab in iTunes and ensuring that the "Sync music" box is checked. In addition to this, if you wish to sync your Podcasts, Games and Contacts, click on the relevant tab in iTunes, check the "Sync" box, and then click on the Sync button at the bottom of the screen.