Javascript - Get checkbox values from a form and get a random result

I have a form that allows multiple checkbox. I want to get a random value of all checked items when the "Generate" button is pressed.

On the one hand I have the form:

<FORM NAME="testform">
<INPUT CLASS="cities" TYPE="checkbox" NAME="mex" Value="mex"checked="checked">México<BR>
<p><INPUT CLASS="cities" TYPE="checkbox" NAME="ita" Value="ita">Italy<BR>
<p><INPUT CLASS="cities" TYPE="checkbox" NAME="usa" Value="usa">United State of America<BR>
<p><INPUT TYPE="button" NAME="button" Value="Generate" 
onClick="testButton"><BR>
</FORM>

But I am a novice in JavaScript and I need help building this exercise.

By the other hand I have no idea how to get the form values. These are the random elements of each checkbox.

var mex=["mexico-df.html","veracruz.html","acapulco.html"]
var ita=["roma.html","milan.html","venecia.html"]
var usa=["los-angeles.html","new-york.html","washington.html"]

I want that if something check México and USA, for example, the result should be only one random value between those included in the above variables.

I do not know exactly how to get the checked values from the form and how to pick a random value from only selected items.

Do you have any suggestions for me?

Sorry if my English is not so good.