First, you need to have a qube to run the web server, and use the Firewall documentation to access it from other qubes: Firewall | Qubes OS
Now, how do you run apache from a qube? There are multiple solutions, with pros and cons:
create a new Standalone qube inheriting from another one, install apache and your service, but you will have to keep it up to date, and have fun with the OS upgrades.
this is the most vanilla setup, but requires maintenance over time
you could create a new AppVM qube, and store everything in /home/, apache could be started from rc.local, you may need to edit its systemd service to allow /home to be used if it’s hardened a bit (there is a knob to prevent reading /home)
this is the most privilege separation setup, but you need to tinker with Apache