If you also wish to display some of your categories and sub-categories in the horizontal menu in your blog you can create 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

Also note that if you do not want all the posts to show on the front page, then you can create a static front page for your site, and also create a “blog” page for your posts to display on, but that’s optional.
http://en.support.wordpress.com/pages/
http://en.support.wordpress.com/pages/front-page/

1.You add the Categories you want to display and arrange them in the order you want.

2. Then you add and drop and drag the sub-categories below the appropriate Categories.

3. You add the Pages you want to display and arrange them in the order you want.

4. Then you add and drop and drag the sub-pages (child pages) below the appropriate parent Pages.

5. You add the custom links you want to display and arrange them in the order you want.

6. When you have everything arranged in the menu that you want you save the menu.

7. Go to the “theme location” module at upper left on the menu page. Select your custom menu name from the pull-down labeled “primary location.” Click the save button in that module.

8. Refresh your browser so that it isn’t possibly pulling a cached page, and view your new custom menu with Parent pages and dropdowns to sub-pages, and Categories with dropdowns to subcategories, and Custom Links.

Troubleshooting – My menu isn’t appearing
Go to the custom menu page in the dashboard Apearance > Menus and in the “theme location” module at upper left select your custom menu from the pulldown and then click “save”.