Search Google for Free Blogger templates. I like to use a site called BTemplates. For demo purposes, I'll demonstrate with BTemplates. Peruse BTemplates until you find a template you like. You can download it right away, or you can click demo to see what your blog would look like in full size. I always recommend checking out the demo first.
When you've chosen your template, click "download". Your template will save to a zip file. Unzip the file and save it to a location where you'll remember. I like to save my templates to a folder called "Blogger Templates".