***DISCLAIMER: This is not a guide on incredible voice automation or anything actually at the level of the Jarvis system from Iron Man***
UPDATE: This is now updated for the many changes in the latest version of OSX Yosemite. Some options appear to have disappeared completely, but most functionality is still there. Take a look and enjoy! Please let me know if anything is still wrong or confusing!
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!
The first step is to click on settings and then click on Dictation and speech. Make sure that you turn on dictation. Next you want to check the small box below it that says allow enhanced dictation. This allows for many more options and offline use which is handy. Once you've done all that you want to choose a button as a shortcut, try and make it easy to remember.