As the process of adding iTunes-purchased videos to your iPod Touch is simple and easy, I won't expand on it here. If that was the extent of your question, then you should visit the iTunes website and explore it yourself. The set-up is meant to be intuitive, and the site is easy to navigate.
Moving on, you can expect to face a longer process when it comes to adding videos from sources other than iTunes. Fortunately, the iTunes software is designed with convenience in mind, so if you're good at following directions, you should be all right at getting video on your device.
Your first step is to create a new folder to store your videos in for now. This will make it easier for you to import them into iTunes. If your videos are already in the correct format (MPEG-4), then all you'll have to do is open up iTunes and use the "add file to iTunes" command to import the videos. Easy.
If your files aren't in MPEG-4 format, then you'll have to take an additional step, but it's also pretty simple on your part. The iTunes program has a built-in conversion tool that you can access by selecting the videos you want to convert to MPEG-4, right clicking, and choosing the conversion option.
Once your videos are all on iTunes and in the proper format, then you can hook up your iPod Touch to your computer and download your movies. All you have to do here is drag your new files to the iPod icon. Depending on the amount and size of the files, it may take a few minutes to transfer them to your device.
Your program will tell you when the transfer is complete, after which you can eject your iPod and enjoy your movies. Enjoy the show!