Pomodoro Clock increment and decrement issue

I am having issues figuring out how to start a timer as well as increment and decrement. Getting the timer from 00 seconds to counting down 60 seconds and decrease one minute after every 60 seconds.Thus far i only displaying the 25 minutes in my chrome browser.I am new to most javascript concepts. Any help will be appreciated.

<span id="get"></span>
var minutes = 25;
var seconds = "0" + 0;

document.getElementById('get').innerHTML = minutes +":"+ seconds;
 function timeDecrease(){
 minutes = minutes -1;