Apache is a lightweight webserver. I'll use it to run this website for example with the help of mkdocs.
On the MagPi website there is an article about setting up an entire LAMP server
sudo apt-get install apache2 -y
Now make it accesible for the user
sudo chown -R <your_username> /var/www/html
And last but nog least, forward port 80 from the modem to your Raspberry Pi. Check the portforward.com website how to do this on your modem/router.
Delete the content from the folder
/var/www/html and replace it with your website.