You may want to gather all of your posts with a particular label (category) into a page and have a tab in a horizontal menu bar below the blog title for visitors to click on to get to that page. The way to do it is to first label your posts with various labels. You will need the links for the various labels (categories) so first go to LAYOUT and add a LABEL gadget to the sidebar. Click on VIEW BLOG to display your blog with the sidebar.
Then go to
Dashboard > Layout > Add a gadget > Add a Page gadget (Note: The HOME tab is automatically already added for you)
You will get a pop up window "Configure Page List". For a proper horizontal menu bar, I would suggest you delete the title "Pages". Then click ADD EXTERNAL LINK (see screenshot below):
In the new pop up window, to Add a New Page, Web Address, type in the title (name) of a particular label (cagetory) you want to add. Then go to your blog, right-click on the listed label (category) in the sidebar, select COPY LINK to copy the web address (url) into the clipboard, go back to the pop up window, right-click in the field for Web Address (URL) and paste.
Continue doing one by one for all the label (category) pages you want to add, then click SAVE.