Do some reading about html5. Descide if your site should be responsive (that is shall it resize to fit various sized clients such as both desktop browsers and mobile browsers well).
If it should be responsive do some reading about that.
My "personal/general" advice:
I think you should get rid of the frames. If you are to use asp.net (not MVC), you could put the parts that are shared between pages in a master page. Menus should be located in a <nav>
element and the navigation menu should be built using <ul> / <li>
elements and styled to look like you want.
There are lots of examples on the web. For example take a look at http://www.initializr.com/ where you can download a template for a new website in the "new style".