Basically, installing a GUI boils down to (as root): pkg install xorg-minimal mate
(replace 'mate' with the Desktop Environment of your choice if different).
Also make sure D-BUS is enabled (there should be a dbus_enable="YES" line in /etc/rc.conf).
Being new to Unix, if you're not familiar with any available text editors, you can just type (as root):
echo 'dbus_enable="YES"' >> /etc/rc.conf
and reboot your VM.
Then, log in as the non-privileged user you've created during installation
and type (only need once): echo "exec mate-session" > .xinitrc
then: startx.
In a VirtualBox VM, it will work without further configuration, except maybe for the keyboard layout if you don't have a US QWERTY one (click System/Preferences/Hardware/Keyboard).
Of course, you'll need to install more applications, depending on your needs. However, before going any further, I recommend you read (at least) Chapter 3 of the FreeBSD Handbook, it's one of the best introduction to Unix I've ever seen.
Note that on bare metal, more configuring and troubleshooting is usually needed and it's a good idea to search the forum for supported hardware beforehand.