I can't remember the first thing I built in Minecraft >.< I know I used to build a lot of houses and things in Classic, though. Then when I bought the early Alpha release I half make a wooden-plank and slab house, ran out of resources and started to expand it up and out as a tree house.
Good times

Hmm, I was clueless, and being a new minecraft player, I was scared shetless of the mobs. At the time, zombies dropped feathers, I think it was 1.5?
And so, I spent the time from when I spawned, to FISTING a wall and made a hole and a tunnel house.
The tunnel was about 2x1 and 15 blocks deep, and it ended into a hole about 4x4x4.
And it had another tunnel.
I didn't know about torches, nor did I know about tools.
So..I spent 4 minecraft days destroying <100 blocks.
The house was pitch black because I didn't know about any lighting. I spent a good 40 minutes following the pitch black tunnels. I confused the tunnels sometimes. The "mining" tunnel eventually led into a cave and I died in there 5 times.
I then made a little hole in the dirt and covered it and made a little bunker and spent an hour in there.

I was 13 and scared of the mobs. Like I was so scared I trapped a chicken and made it my buddy to protect me.

Back in classic minecraft, the first thing i ever built was a tiny 5x3 brick house right by a shoreline, complete with 1 block wide wooden piers .
And in survival minecraft, my first build was a wooden hut.

The first thing I built was a crafting table

hahah nice stories everyone so far tell ur friends to tell theirs as well!

A 5x5 Dirt House o_o

A dirt wall.

I built a hole. beat that

I carved into a mountain and used some of that dirt to make a door. I stayed there and opened the wall every couple minutes to see if it was light out haha the good ol' days

I first found a witch hut. I made a nether portal and made the exact same hut. Only with pillar quartz and quartz. Oh and no flower pot. It was a wart. I still have it.

My first house was pretty good. It was big and had a farm. Made from wood & cobble. Looked good.

I made a house carved from a mountain,but it had nothing inside

The first thing I ever built...I remember it well. I kinda wish I still had it--but it disappeared with an old computer of mine.

It was a house I had built into the side of a huge mountain. I didn't know how to use any mods, and I felt like Creative was cheating, so I did it all in Survival--but my crowning glory was the huge 200+ track minecart ride that ended in the Nether (back when, I believe, you only got TWO track pieces for all that iron, instead of SIXTEEN). I made it so it curved around lava, went under water, and then led into this huge cave filled with water features and stuck a portal to the Nether in it.

I built a house of obsidian, glowstone, netherracka and lapis in creative... I thought it looked awesome (it was 4x4 )

A wooden block I mined (I didn't know the controls so...) from a tree before I accidentally deleted the world save

The first thing I built was a wooden house under a tree, pretty spacious. I killed my first creeper without it exploding in my face and it gave me what gunpowder was called before, Sulphur. Then I walked out and a duo of skeletons killed me!

The First Thing i Built...Well the first thing i built was a Nice....Small...Cave Home. it was quite cozy. until Beta Came. when alpha transitioned into beta it corrupted my world

A house made of Iron...it was in creative.