What If You Don't Like The Template Choices That Blogger Has?
Most of these other templates involve a simple "copy and paste" method to them - which means that you are going to have to delete your blogger.com template and paste the new template in. Again I should warn you that you should make a backup copy of your original blogger.com template - just in case.
As far as that "blank space" on the side of your blog is concerned.....then there may come a time when you will find something interesting that you will want to add to the side of your blog - that is what that "blank space" is for.
As you can see from my own blog (and on other peoples' blogs as well) there are a lot of other things that you can add on the side of your blog that others (and yourself) might find quite interesting and useful.
There are a lot of widgets that are freely available online. Widges are pieces of information that one can add to the side of their blog (or website). Most of them involve you simply copying and pasting their code.