Hi,
You have to need some type of practice of html designing and css.
I am giving you very basic trick as per my knowledge.
I have designed many web pages in asp.net
You have to fix your all pages as per following way
in div i fixed width of pages in pixel after in that you can take width in percentage.
<body>
<form id="form1" runat="server">
<div style="width: 990px; margin: 0px auto; z-index: 10">
</div>
</form>
</body>
I think this answer can help you..