You want to watch tutorials. Go to youtube, watch Jamziboy or Jeracraft (or Dukonred if you are getting more advanced/want to use the conquest mod). You need to learn and understand how building works.
You want to have certain tools to create better worlds. Worldedit is a great tool. It makes building far more easier. Play around with Worldpainter too. Tutorials is how I started, just watch. Just try to see and understand how and why they are doing certain things.
Hey there! I saw your post a few days ago but didn't have any suggestions until I stumbled across this video today. It seems to have really good advice. It might help you out in figuring out how to build better. Good luck!