Need some help with where to start

Greetings!

So I am trying to make a game called Imprisonment. The very first starting section of the game is in a cell. Now, I know what to use for the cell door, but I don't know what to use for the walls, floor, ceiling, and furniture that will be within the cell. What I am asking is, where do I start? I want it to be good. Let me make an example...

There is a game called Slender, and a friend of mine said he has Unity, but hasn't made a game yet. I would like the graphics to look like that. Me and him are trying to understand where to start. Now my friend has been working with 3D graphics for quite some time, and he told me not to just implement a wall into Unity and such so...I need some clearance.

In order for me to make a legit game, with good graphics, and smooth game play, where do I start? What do I do? I have textures but do I edit them in Blender? Help...please.

Thanks in advance.

-@-

Alright so, I must not be looking well enough. I'm trying to find out how the polygons work in Unity.
If I have a wall, and I put them together, will Unity render the side that's touching, or ignore that part? I'm trying to understand what I can do to shorten polygons the amount of polygons. Will it be easier to make a level one big structure, or separate it? Any feedback on this?