Your objectives are to install VirtualBox on your Debian server, and then install CentOS 7 as a virtual server inside of VirtualBox.

First, install VirtualBox. You can install a full version of VirtualBox from VirtualBox.org, or you should be able to install it from a Debian repository. (If you see "OSE" in the name, it refers to the "Open Source Edition", which is fine.)

Next, pull down an ISO of CentOS 7. I have cached a copy here, but if you are interested in seeing all of the different possibilities for ISOs and downloads, look at the download area of CentOS.org.

Now use the "create a new machine" option on VirtualBox; use a VDI (not a real partition) and this disk image doesn't need to be very large (30 gigabytes is more than sufficient; the "minimal" installer imposes a circa 10 gigabyte minimum). Give the virtual machine only one processor and 4 gigabytes of memory.

A journal is not due for this assignment. Please let me know when you finish, and I will verify your work (so please leave your VirtualBox CentOS 7 instance up for us to look at.)