Summarize the following:
Go to https://www.ubuntu.com/download/desktop in your computer's web browser. You can download the Ubuntu disk image (also known as an ISO file) here. You'll find it near the bottom of the page. It's a green button to the right of your selected version of Ubuntu. Doing so takes you to the support page. This link is in the bottom-left corner of the page. The Ubuntu ISO should begin downloading immediately, but if it doesn't, you can click the download now link at the top of the page. While the Ubuntu ISO downloads, you'll have plenty of time to set up your virtual machine in VirtualBox.
Open the Ubuntu website. Scroll down to the latest version of Ubuntu. Click Download. Scroll down and click Download Now. Make sure that Ubuntu begins downloading.