How to write jQuery code or call jQuery function in onclick attribute

It doesn't work as functions called from the on* event attributes need to be within scope of the window. Your code places the function within the jQuery document.ready handler instead. Try this:

function myFunction() {
  alert("sss");
}

<div onClick="myFunction()">Click me</div>

You should note though, that using on* event attributes is considered outdated for this reason, amongst many. You should instead use unobtrusive JS code to attach your event handlers, either in native JS:

document.querySelector('div').addEventListener('click', function() {
  alert("sss");
});

<div>Click me</div>

Or jQuery, as you seem to be using it already:

$(function() {
  $('div').on('click', function() {
    alert("sss");
  });
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div>Click me</div>

First Go and read tutorials of html and Jquery at some tutorial websites like This or thisone.

and answer to your code is

HTML

<button onclick="myFunction()">Click me</button>

Javascript

function myFunction() {
    alert('Hello');
} 

Assuming you included JQuery libraries.