So, a few weeks ago, I started playing Minecraft too.
After surviving my first few nights and digging holes into the ground, I decided to go for the big one right away.
My first real home on my first world:
The outside is built entirely out of obsidian. Inside, I used generic cobblestone, so I can actually see where I'm going in there.
I was quite lucky with the obsidian. Found a lavapatch on the surface not far from my spawnpoint, and a huge lavalake at the bottom of my mine.
That made up for about 2/3 of the tower. Got the remaining obsidian by using one of those obsidian generators that you can build.
After I was finished, I marveled at the view from above, when, suddenly, snow started to fall.
Imagine my surprise when I realised that I had built my version of Orthanc exactly on the border between a cold and snowy region and a warm, green landscape with rain.
The region behind the tower:
The region at the frontside: