need some help with a timer script

Unity Engine
1

I need some basic help on a timer script
it should be something like this:

When all enemy is destroyed
start timer for 15 seconds
then send next wave of enemy

But i’m kinda new with the whole unity program…
thanks in advance for the help

2 
var timer : float = 0.0;
//start Timer Here
timer += Time.deltaTime;

if(timer >= 15){

YourFunction();

}

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