How should I display fields for a content type?
You can use display suite module to theme your node.


  Display Suite allows you to take full control over how your content is
  displayed using a drag and drop interface. Arrange your nodes, views,
  comments, user data etc. the way you want without having to work your
  way through dozens of template files. A predefined list of layouts (D7
  only) is available for even more drag and drop fun!


If you want to theme your node using code, then you should create template for your node. Check this link for reference.