where can i access the code on my existing blog to apply custom design to it?
You cannot build or use your own custom theme built from scratch at WordPress.com. The only themes we can use are those found here > Appearance > Themes This is a multiuser blogging platform. All blogs wearing the same theme are using the same underlying template and we cannot access the underlying files and edit templates or themes. http://en.support.wordpress.com/themes/editing-themes/
There are two free themes, Sandbox and Toolbox designed to allow maximum flexibility for CSS editing and creating new “skins”. Toolbox is more modern and includes a lot of support for CSS3 and HTML5, so it is really only limited by your skills and knowledge.
If you have CSS editing experience the upgrade will allow you to stylize the appearance of themes you find here > Appearance > Themes, but will not allow you to change the functionality, by editing the underlying template. It does not allow you to remove footer links to the theme designer or to WordPress.com. Also note that CSS is theme specific, so you cannot use the CSS stylesheets from other themes with the themes at WordPress.COM. http://en.support.wordpress.com/custom-design/custom-css/
See also > WordPress.com vs. WordPress.org: The Differences > http://support.wordpress.com/com-vs-org/
The answer to your second question follows. You cannot post to pages. Pages are static and sit outside the blog structure. With wordpress, you use categories to organize posts into subjects and then you can add those categories to your navigation using a custom menu.
Staff have provided help for those who want to structure their blog like a website. > http://en.support.wordpress.com/using-wordpress-to-create-a-website/
What’s critical is:
(1) a clear comprehension of the differences between pages and posts > http://en.support.wordpress.com/post-vs-page/
(2) a clear comprehension of the fact that there is only one dynamic page in a blog for posts and we cannot post to more than that one page. But we can create the appearance that we have posted to more than one page.
We organize our posts by assigning Categories to them. When we publish a post it automatically appears on the running page for posts and also on the Categories pages and Archives pages. Note: There must be one published post in each Category in order for there to be anything to display.
A custom menu allows you to display Categories with drop-downs to sub-categories in tabs along the horizontal navigation where normally only Pages tabs are displayed. If you wish you can also include Pages with drop-downs to sub-pages and/or custom links in your custom menu as well. You are in charge of what appears in a custom menu. You choose the order in which to display any and/or all the foregoing in your custom menu. You choose which to display and which to hide. http://en.support.wordpress.com/menus