Hello and welcome,
In order for this to be as simple as possible I will outline a minimal setup.
Install things (and be refreshed/synced/up-to-date);
sudo pacman -Syu qemu-desktop virt-manager
Add user to group;
sudo usermod -aG libvirt $USER
Afterwards you can start/enable the socket or service;
systemctl enable --now libvirtd.socket
And “Virtual Machine Manager” should then work.