I am in the process of finishing my basement and I want to install a shower in the bathroom. It is already roughed in for a shower, there is a 12x12 square hole in the cement and about 1 foot below the floor is a capped horizontal pipe. I want to install a 4 piece shower 36x36.
1.) What do I all need to install the shower drain?
2.) Is there a P-trap under the cement or do I need to install that as well.
Thanks.

You aren't roughed in for a shower. The square hole is a tub dap out. We rough in out tubs 15 1/2" off the side wall and 1 1/2" off the front plate. To install a 36 X 36" you will have to move the trap and raiser and yes there's a trap down there. Now they do make a 5' manufactured base that's made for installing in a tub rough in. They're kind of pricey but I can direct you if you wish. Good luck, Tom

I called the builders of the hosue and they said that the square hole in the concrete is for a shower, or a tub, or a combo unit. With that being said:
What do I all need to install the shower drain?

We don't dap out for a shower installation here. You ask, What do I all need to install the shower drain?"
That depends on what type of shower you intend to install. A custom tile shower or a manufactured base and inclosure one.
Can you give me the measure of the trap raiser in relationship to the walls? That will depend in large as to what you can install. Regards, Tom