lol... you can't figure out you need an array and random function? this isn't anything to do with web language but a basic concept of programming in todays world.
Code:
var ImaRay = [
"Set The",
"Array Length",
"For Random"
];
var rngesus = Math.floor((Math.random() * cars.length));
Notice what I'm doing, I'm setting the items I want into an array then setting another variable of using the math.floor function to display the Random function which the Random function is defined by multiplying Math.Random(unknown) by the length of the Array (the number of items you have) and then simply displaying it like so
Code:
document.getElementById("Generator").innerHTML = ImaRay[rngesus];
<p id="Generator"></p>