You might want to start by modifying the [u]Blink Example[/u], something like this:
digitalWrite(LED_BUILTIN, HIGH); // turn the LED on (HIGH is the voltage level)
delay( random(5000,15000) ); //Hold LED on for between 5 & 15 seconds
(There's actually a 1 millisecond "error" in that code. 
)
Now normally... We like to avoid the use of delay(), especially delays longer than a couple of milliseconds. But in your case, you don't need to do anything during the delay time (I think) so you'll be OK.