iPod accepts only MP4 format video, please make sure your video is compatible with iPod. If your vide is MP4 video, please follow these steps:
1. Open iTunes and click "File" to add your videos to iTunes library.
2. plug in your iPod to computer, then iTunes will detect your iPod as a device.
3. Select your iPod device on the left panel of iTunes, click "Movies" tab, check the videos you want to sync and click "Apply", done.
If your video isn't MP4 format, you need third party software to convert video to MP4 video,I always use Aimersoft iPod Converter Suite to convert video to mp4, and then transfer the video to my iPod through iTunes, my friends recommend this software to me, it works really nice, you may download it for a try
http://www.aimersoft.com/ipod-converter-suite.html