There are probably tons of hubs (articles) written about how to blog and how to add banners to blogs. You can do a search in the field on the top right of this page and search for hubs.
Also, check out You Tube. Do a search about how to blog, etc. I bet you'll find lots of step by step videos on how to blog. Now that I think about it, Blogger has it's own channel on You Tube. I went over and got the link for you. Here it is. http://www.youtube.com/user/BloggerHelp
Hope this helps. Best wishes.