If i created element in javascript and how call function when i click on the created element?

either you need to add the click event when you create the element

function elem() {
    var div = document.createElement('div');
    document.body.appendChild(div);
    div.id = 'myDiv';
    div.innerHTML = 'This is created div element with javascript';
    div.onclick = function() {
       alert('Hello');
    };
}

or you need to listen for the click event with event bubbling with an element that is a ancestor.

document.getElementsByTagName("body")[0].onclick = function (evt) {  //there are better ways to add an event
    evt = evt || window.event;
    var target = evt.target || evt.srcElement;
    if(target.id==="myDiv") {
        alert("hello");
    }
};

Add the event handler when you create the new element. Otherwise the script will execute and attempt to assign the event handler before the div is attached to the DOM.

     function elem() {
        var div = document.createElement('div');
        document.body.appendChild(div);
        div.id = 'myDiv';
        div.innerHTML = 'This is created div element with javascript';

        div.onclick = function() {
            alert('Hello');
        }
     }

JS Fiddle: http://jsfiddle.net/Mve2T/

Your myDiv code should exist insdie the elem function you can use also addEventListener() instead of 'onclick' for binding the handler on particular event. Something like this.

function elem() {
    var div = document.createElement('div');
    document.body.appendChild(div);
    div.id = 'myDiv';
    div.innerHTML = 'This is created div element with javascript';

     var myDiv = document.getElementById('myDiv');
        myDiv.addEventListener('click', function (){
        alert('hello'); 
    });
 }

Demo

you better add your event in your elem function like:

<script type="text/javascript">
     function elem() {
        var div = document.createElement('div');
        document.body.appendChild(div);
        div.id = 'myDiv';
        div.innerHTML = 'This is created div element with javascript';
        div.onclick = function() {
            alert('Hello');
        }
     }
</script>

your DEMO