Assuming a simple home setup, you have a computer running apache and accepting requests on port 80. It sounds like apache is running and I presume you can access it using the local loopback address,
http://127.0.0.1. You don't need to specify port
80, but it won't hurt.
Your server will also have an IP address on your local network, perhaps something like
192.168.1.12. This is a private IP that only has meaning on your home network and was either provided by your router (dynamically using DHCP), or has been statically defined by you. You can test whether your server accepts requests through it's firewall by using a 2nd computer on your network and browsing you server's private IP,
http://192.168.1.12 in my example.
Your router will have both a public and private IP address. To access your web server, you will have to tell your router to forward all requests received on the public IP address to your private server IP address.
You need to use your router's public IP address to access your web server from the internet. If you don't have a static public IP, then your ISP assigns one dynamically. You will need to use a dynamic DNS service if you wish to access a dynamically assigned IP address consistently.