Apply Template to Specific Field within Form

Hi,

I am VERY new to ServiceNow and JavaScript so bear with me.

I am trying to apply a template to the 'Description' field within my form. I need the template to appear when the user clicks into the field so that they can fill out more specific information about their incident.

I have this so far:

function onLoad() {

  if(g_form.isNewRecord()){

  applyTemplate('gm_story');

}

}

How do I add this template to the description field specifically and how do I get it to appear when the user clicks in to it?

Thank you,