Someone just mentioned to me that they didn't know how to add a Blogger navigation bar. Well, it's quite simple. Most people seem to go for the horizontal version. Some for the vertical one on the sidebar.
Okay, go to Design>Pages. Click on Pages. It will say New Page.
The first thing you see says Blank Page. You could have an About Me page here, and write about yourself. Whatever you want. It is a blank page.
Say you want to just have a Page go straight to a website, or something on your own blog. Click the bottom one. You will be linking straight to another site.
I think you are allowed 10 Pages on Blogger. You probably won't need that many. Especially if you want it to run horizontally underneath your header.
On the right, you will see Show Pages As. Underneath that you will see Top Tabs, under that Side Links and under that Don't Show. For a horizontal navigation bar, you would of course choose Top Tabs. If you want it in your Sidebar, you would choose Side Links. And if you want to start a Draft for a Page, but don't want to use it yet, you would choose Don't Show.
As you do these Pages, either Save or Publish. If you Publish, it will automatically go to the type of Navigation Bar you have chosen. If you have chosen horizontal (Top Tabs), publishing it will put it automatically under your header.
Keep adding Pages until you have what you need for your Navigation Bar. You can always go back in and edit your Pages, change them around. Whatever.