how can call any method in c# from javascript ?

You never ever really call any server-side method from Javascript, which is always called on the client side, by a browser. (I don't count rare cases of server-side Javascript — you hardly use it if you have tagged "ASP.NET").

All you can do is to send some HTTP request to the server and get HTTP response. The resource pointed by the URL used in such request should be programmed to interpret such request and provide appropriate responses. From the Javascript, it is done using Ajax: http://en.wikipedia.org/wiki/Ajax_(programming)[^].

A very convenient way of doing so is jQuery Ajax: http://api.jquery.com/jquery.ajax[^].

If you need to learn jQuery (highly recommended), please see:
http://en.wikipedia.org/wiki/JQuery[^],
http://jquery.com[^],
http://learn.jquery.com[^],
http://learn.jquery.com/using-jquery-core[^],
http://learn.jquery.com/about-jquery/how-jquery-works[^] (start from here).

—SA

Make C# method Static and Tag it with [WebMethod] attribute
like this :-

[WebMethod]
public static void TestMethod()
{
}

javascript code
=================

<script>
    function test(){
        alert(PageMethods.TestMethod());
    }
</script>

You can call as many functions inside your javascript.
You need to define the function, may be outside the document.ready and then can call anywhere you like . Below is a small snippet:

$(document).ready(function(){
$("#ANYID").click(function(){
MyFunction(parameters);//with parameter
MyFunction2();//without parameter
})

});
function MyFunction(parameters){
//do something
}

function MyFunction2(parameters){
//do something
}

It will help you Calling Javascript function from server side[^]