how do I get it out on the internet?...
...Basically how do I attach a URL to my blog
Here's the short version...
You need a domain name that resolves to your residential ip address. That can be problematic, because most isp's issue residential ip addresses dynamically, meaning that they change every now and again.
You can look into DynDns or a similar service (and an updater) to circumvent those issues.
You need to assign a static internal network address (ie.. 192.168.x.x) within your own home network, to your Mac. You then need to configure your router to port forward (at least) port 80 to that static ip address, so it can receive requests from the WAN.
You need to install WordPress using the actual domain name you're going to use, rather than using "localhost" in the WordPress addresses in order to make it available to the WAN. ( Because all computers are "localhost", requests for a localhost address from the WAN will always result in an error ).
You then need to configure the Apache host container (default, or virtual) to resolve your domain name to the directory where WordPress has been installed.
None of this even begins to touch on the security implications of opening a home computer to the WAN for use as a web server, of course...
Alternatively, you might look for some inexpensive hosting.