In the video you linked to, right around the 4:46 mark, there is a screen that gives you options for installing ubuntu. The second option, "Erase disk and install Ubuntu" will wipe out your entire disk drive (including whatever data you have in Windows) and install Ubuntu, using the entire drive in the process.
If you don't have any data you want to retrieve from your existing Windows installation, or you have already backed up your data somewhere else, then this is the option to choose if you want to single-boot Ubuntu.
On the other hand, I strongly recommend you hold on to your windows installation by dual-booting, as shown in the video. As a beginner, I'm not sure you have fully explored Ubuntu yet, and may regret removing Windows from your computer for the following reasons:
- Although most windows programs have equivalent programs in linux, these are usually not exact replacements. You may find yourself having to use windows specific apps.
- Some devices on your computer may or may not be fully supported by Ubuntu, or may require some tinkering to get them to work. For example, it is possible that after installing ubuntu, your network device doesn't automatically connect to the internet upon boot. Unless your have an alternative way to go on-line, you will be caught in a chicken-and-egg situation of needing to go on-line to get help for your connection problems. It is not a pleasant situation to be in.
- Even if your present needs are totally supported by Ubuntu, a situation may arise in the future that would require you to use a proprietary Windows-only program. For example, you switch jobs, and your new company requires you to use a Windows-only program.
Note that you can always remove Windows later on, after you have already explored Ubuntu in its entirety.
At the very least, make sure that you have the means to reinstall Windows should the need arises, either by making a back-up of your current Windows installation, or having a Windows installer/recovery disk.