Add this property in your page directive.
<%@ Page MasterPageFile="myMaster.master" %>
The you need to remove everything apart from the contents of the form tag.
<asp:Content id="ContentPlaceholder2" runat="server" ContentPlaceholderId="MasterID">
paste content of form tag
</asp:Content>
ContentPlaceholderId id must be id of ContentPlaceHolderfrom master page.