IN ONCLIENT CLICK OF BUTTON YOU CAN USE LIKE THIS
<script type="text/javascript">
function fnalert()
{
alert("Hi, this is alert...|");
}
</script>
<asp:Button OnClientClick="javascript:return fnalert();" Text="Click Me"/>
YOU CAN USE THIS in CODEBEHIND LIKE THIS
Page.ClientScript.RegisterStartupScript(this.GetType(), "aa", "<script language='javascript'>alert('HI THIS IS ALERT');</script>");