I am pre-assuming that you are adding this in your user control or visual web part.

You can add button click event in various methods:

C#

Your HTML part:

<asp:Button id="Button1" Text="Click here for greeting..." OnClick="GreetingBtn_Click" runat="server"/>

In code behind file there is one method for click event:

protected void GreetingBtn_Click(Object sender,
                           EventArgs e)
    {
        // When the button is clicked,


        //You can do your logic here
    }

JavaScript

Your HTML part:

<asp:Button id="Button1" ClientIDMode="Static" Text="Click here for greeting..." onclientclick="btnOneClick" runat="server"/>

Your script code for click event:

<script type="text/javascript">
     function btnOneClick()
     {
         var updatedLabel = document.getElementById('<%=inputLbl.ClientID %>');
         updatedLabel.innerHTML = updatedLabel.innerHTML + "1";
         return false;
     }
</script>

JQuery

Your HTML part:

<asp:Button ID="btnSummary" runat="server" Text="Next" />

Your script code for click event:

<script type="text/javascript">
function pageLoad(sender, args)
{
     $("#<%=btnSummary.ClientID %>").click(function() 
    {
        alert("1");
    });
}
</script>