This is asked many times on asp.net forums,
1. How to call Server Side methods or functions using JavaScript in ASP.Net
2. How to call Server Side methods Client Side in ASP.Net only
Initially I explained the same using jQuery in the following article.
But the issue is that many developers are not well acquainted with jQuery hence now I am explaining the same using ASP.Net AJAX PageMethods.
ASP.Net AJAX ScriptManager allows you to call Server Side ASP.net methods from client side without any PostBack using PageMethods. Actually it is an AJAX call to the server but it allows us to call the method or function defined server side.
Enabling PageMethods
The first thing you need to do is add a ASP.Net AJAX ScriptManager to the page and set itsEnablePageMethods property to true as shown below
<asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true">
</asp:ScriptManager>

HTML Markup
<form id="form1" runat="server">
<asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true">
</asp:ScriptManager>
<div>
Your Name :
<asp:TextBox ID="txtUserName" runat="server" ></asp:TextBox>
<input id="btnGetTime" type="button" value="Show Current Time" onclick="ShowCurrentTime()"/>
</div>
</form>

As you noticed above I have added a textbox when user can enter his name and a HTML button that calls a JavaScript method to get the Current Time.
Client Side Methods
<script type="text/javascript">
function ShowCurrentTime() {
PageMethods.GetCurrentTime(document.getElementById("<%=txtUserName.ClientID%>").value, OnSuccess);
}
function OnSuccess(response, userContext, methodName) {
alert(response);
}
</script>

Above the ShowCurrentTime method makes an AJAX call to the server using ASP.Net AJAX ScriptManager PageMethods and executes the GetCurrentTime method which accepts the username and returns a string value.
Server Side Methods
C#
[System.Web.Services.WebMethod]
publicstaticstring GetCurrentTime(string name)
{
return"Hello " + name + Environment.NewLine + "The Current Time is: "
        + DateTime.Now.ToString();
}

VB.Net
<System.Web.Services.WebMethod()> _
PublicSharedFunction GetCurrentTime(ByVal name AsString) AsString
Return"Hello "& name & Environment.NewLine &"The Current Time is: "& _
            DateTime.Now.ToString()
EndFunction

The above method simply returns a greeting message to the user along with the current server time. An important thing to note is that the method is declared as static (C#) and Shared(VB.Net) and also it is declared as Web Method unless you do this you won’t be able to call the methods using ASP.Net ScriptManager PageMethods.
The figure below displays the output displayed to the user when the button is clicked
Calling Server Side Methods JavaScript and AJAX in ASP.Net
You can download the code in VB.Net and C# using the link below
CallingServerSideMethodsusingJavaScript.zip