There are several ways to customize a blog on Blogger, and most of them are pretty simple.
To begin, head to the admin page and click the "Template" link on the left side. Hit the customize button. This takes you to the template designer page. You can choose from a variety of templates built into Blogger, and you can choose lots of different backgrounds to change how the template looks. You can even upload your own background image and header image.
The template designer has simple controls for adjusting the width of different elements of your blog and shift around the layout of blog elements, and the advanced tab lets you change the colors of every last element of the blog. All the changes you make can be viewed as you make them, before you finalize them and make them active on the blog itself.
One of the most powerful ways to customize a blog at Blogger is to upload a custom blog template. Information on a blog's design is stored in an xml file. This allows you to back up your blog design by saving the xml file to your computer.
But there's a more exciting way to use xml files with Blogger. There are tons of free custom blog templates designed to use with Blogger scattered around the web. When you download one of these templates, you'll get an xml file (usually contained in a zip or rar file). To install one, go to the Template tab of the admin panel. In the upper right you'll find a button called "Backup/Restore." Hit that button.
Where it says, "Upload a template from a file on your hard drive," hit Browse and find the xml file wherever you saved it on your computer. Then hit "Upload" and you're done. It only takes seconds.