***DISCLAIMER: This is not a guide on incredible voice automation or anything actually at the level of the Jarvis system from Iron Man***
UPDATE: With the most recent release of OSX many of these options no longer exist. Much of the voice control is still intact but just consolidated. I may Update this sometime in the next couple weeks when I have time but if not just know that the idea is the same but a lot of the steps below are no longer accurate/needed.
However, this simple guide will show you how to adjust some simple settings on your mac in conjunction with automator to give you pretty good basic control. By the end of this, you should be able to go to frequently visited websites, start music playing, navigate between and within programs and even ask basic queries such as what the weather's like. If you know how to program in applescript you can really do a lot with the built in voice command system that the mac provides, far more than me, but no programming is necessary for this. So without further ado, let's get started!
Step 1: Turning on Speakable Items
The first step is to click on settings and then click on accessibility. On the left you should see something called Speakable Items like in the picture I have posted. Make sure that you turn it on. You can also adjust options such as a sound acknowledgement of a command or a spoken command acknowledgement. I have a sound acknowledgement cause it makes it clear when a command has been heard and easier for debugging.
Next, you should see a tab across the top that says listening key, click on it. Then you can change the key to whatever you want to basically turn the system on and off. Then you need to change the listening method to listen continuously with keyword. You can see my keyword is Jarvis but you can set yours to anything. You also don't have to actually require it before every command but again I do. Otherwise it might hear random sounds or songs or anything and try and execute a command.
Finally click on the commands tab across the top. Make sure all these are checked so it can control everything. Now, that's it! the voice command system is initially set up. Now for the fun part, customizing it!