This can be made in survival or creative mode, though it would be very hard to get enough resources in survival. And now, I present to you, the Minecraft Treehouse!

Step 1: Trunk

Make a tall pillar out of wood. It doesn't matter how tall but don't go above the clouds. then make two pillars diagonally touching the first. They have to be the same height. There will be a grass block in middle of the pillars. Remove it and place a wood block there. Make a pillar diagonally touching the two new pillars. Now add four more pillars, making a square trunk. Then go to the front and remove two wood blocks. remove the grass block beneath it and place a wood block. Now put a door there. Go inside your hollow trunk and make a ladder.
UPDATE: You can also use vines as stairs

Step 2: Leaves

Use the leaves that come from the same type of tree that that the wood was. you can do any type you want but make sure there is a hollow the size of a room inside. My room was three blocks tall and the pictures are the bottom and top of my tree.

Step 3: DECORATE!!!

For your bed and anything on the wall you will have to remove leaf blocks and swap them with something else. I used green clay. Set your bed on top of the new blocks. Now dig out the new blocks and put leaf blocks in their place. I know this seems like a stupid step but you can't place a bed on leaves. Do the same for anything on the wall but don't remove the new block(s). The last two pictures are my finished inside.