You will choose a template when you first setup your site as part of the Setup Wizard. However you can change your mind at any point and switch to a different template.
To do this log into your site's Control Panel Then click Website --> Change Website Template.
Once you have finished the system will take some time to create new thumbnails for all of your product images, as the new template may display different sized thumbnails to the old one. Don't worry though this will happen automatically for you.
The new template will take effect as soon as you publish your site.