You can not write server-side event on this page. You have to post your data to a form that can interact with the server(an aspx page in your case)
change your line of code:
<form action="" method="post" class="niceform">
to
<form action="Enter.aspx" method="post" class="niceform">
Then on page load event of Enter.aspx page write:
NameValueCollection frm = Request.Form;
string userName, password;
userName = frm ["UserName"];
password = frm ["Password"];
This way you can access the entered values on your HTML form and perform the operation on server.
Remember, to pass the values of username and password you must use "name" attributes. In your HTML name attribute is blank