Try to get away from the somewhat rigid designs of old buildings and invent your own architectural style.

what helped me gain 90% of my skills are the following things: Creative plots servers with a constructive community (not like Edawg and similar that are filled with skin competitions), asking feedback regularly, watch tutorials like the above person said, and practise a LOT. It took me 3 years to get to my current level, but thats because I kept taking Tekkit hiatuses of several months. Others have done it faster.

Anyway, there are a couple of essential elements to building. I'll make a short list including the meanings.

• Detail: The most commonly referred to element, it involves making plain stuff not plain. There are 2 ways to do it, Natural detail and structural detail.
  Natural detail is detail that makes sense, like a button as doorbell, tools hanging above workbenches in a workshop, lanterns, stuff like that.
  Structural detail uses blocks, stairs, slabs and fences and the likes. It does not have to make sense, its just there to make the build look more interesting (though it making sense is definitely a big pre). This is most used and easiest to learn, since it does not require large amounts of realism in a game that doesnt lend itself for that.
  
• Depth: A less commonly known part of minecraft building, it involves making your build look less flat by putting the detail on separate layers. its kinda hard to explain more without pictures, so I wont try.
  
• Theme and palette: This is about the blocks you use. For medieval-ish styles, wood and stone type blocks will be the obvious choice, but white wool and brick may also play a role. For more moderny style houses, quartz may be of interest. Fantasy isnt really defined since its a very broad term and basically means anything that doesnt fit into other categories.
• Shape: The geometrical shapes that your building is based off. A good build has more than 1 geometrical shape, and is definitely not just a box with a pyramid roof.

These two YouTube channels may help give you some ideas.
https://www.youtube.com/user/lorddakr

https://www.youtube.com/user/Madnes64

I am the leader of Prestige Builds, (well known build team)
I'd be willing to help you learn!
my skype: fuzoimc

finish-last-builds-for-huge-server-t486746.html

My friend is looking for builders to help him in projects, and in return he is teaching people how to build

If you think im just trying to advertise for this server, here is his old post made just to train builders, not for a server. Do not post there as it is inactive.

some-builders-who-want-learn-from-t473977.html

You must FEEL the building.

Taste the building.

BE THE BUILDING.

Then move the building with your mind.

I'd have to agree with what GuyThatHasNoLife says.

To get you started however, a few tips: depth, color, texture, don't lose sight of your theme, clay blocks are a god-send, and quartz can be a very versatile block (be careful with it as it has a very pure texture in default).

Other than that, you just have to experiment, honestly. Use builds you see on here for inspiration.
Happy building!

Build to learn, Don't learn to build. That is how a lot of people learn how to build better