Here's my main blog, with lots of tabs across the top:
Make sure you are logged into your blog and click on 'Pages' on the left hand menu. This is where you can create the pages that you want along the top. You have two options: a static page that you write the content for; or a link to another webpage.
Click on 'new page' at the top and it will offer you these two options. You can have a mixture. Select which you would like for your first tab. If you choose to add a link to a webpage, add the title you want to appear on the tab (e.g. 'shop') and then copy in the link - make sure you don't end up with two http bits! Click save to create the page.
To add a static page select 'blank page' and it will take you to an editing screen that looks exactly like the post one. Writing a page is exactly the same as writing a blog post. If you're not sure what all the buttons at the top do find out here. Write the content for your page, e.g. a few paragraphs about you, and then click 'publish', just as you would a blog post. Make sure the title is what you want displayed on the tab.
Continue to add as many pages as you would like. If you click on 'pages' on the left hand menu it will show you a list of all the pages you have added. You can change the order of the pages by dragging them into position. To have the pages show across the top of your blog, where it says 'show pages as', select them to show as top tabs.
Then click on layout on the left hand menu. Under your header, click on 'add a gadget' and scroll down until you see the pages gadget. Click on the little plus icon and this should add your pages to your blog. Make sure you save, then view your blog and admire your new tabs across the top.