A template defines how your Blogger powered blog will look like, including colors, borders, sidebars, fonts, headers, footer, headlines, etc. By default Blogger already provide you with some templates to choose from. If you want to change your current template with one of the official templates, here are the steps:
1. Login to blogger.com, visit Dashboard >> Layout >> Pick New Template.
2. Choose the template you like by marking the name.
3. Click Save Template and done.
If you want to use custom template (not provided by Blogger), here are the steps:
1. Download a template (in form of xml file) to your hardisk. If you want to start looking for a new template, I suggest Deluxe Templates for its handsome support in case you run into problems.
2. Login to blogger.com, visit Dashboard >> Layout >> Edit HTML.
3. Use the browse button to find the xml template you have downloaded.
4. Click upload.
5. Confirm sidebar reset if asked.
6. Save and done.
Tips: changing template resets your sidebar widgets, back up them into text files first if you feel necessary.
Enjoy your new template :D