Go to the magnifying glass icon at the top right of your screen. Type in the words "Terminal" and click on the Top Hit, "Terminal"
You should now see a blank Terminal window:
Type in the words "idle" (without quotes) and hit return:
Idle should start up. It looks like this on my machine:
You may want to increase font size. On my 15" MacBook Pro, the default is pretty tiny. Go to the "Options" menu and click "Configure Idle" as follows:
Now increase the font size to say, 16 point, and then click the "Apply" button at the bottom of the screen:
Okay, now you're totally set up. Feel free to start using idle!