Place your asp/HTML controls inside the ContentPlaceholder of the Page . Your page should look like this
<%@ Page Language="vb" AutoEventWireup="false" CodeBehind="Default.aspx.vb" Inherits="WebUI._Default" MasterPageFile="~/Site1.Master" %>
<asp:Content runat="server" ID="head" ContentPlaceHolderId="head"> </asp:Content> <asp:Content runat="server" ID="content" ContentPlaceHolderId="ContentPlaceHolder1">
<h1>Your content</h1>
</asp:Content>