How to know what random number is chosen

Well I've determined what the random is by declaring a variable like this:

var RandomPick = Random.Range(0, P1Deck.Length);

var CurCard = Instantiate(P1Deck[RandomPick], CardLocations[LocationNumber].transform.position, CardLocations[LocationNumber].transform.rotation);

But now the question is how to remove it. I've tried RemoveAt,

like this: P1Deck.RemoveAt(RandomPick),

but I'm getting this error:

'RemoveAt' is not a member of 'UnityEngine.GameObject[]'.

GameObject[] is a built-in array. One of its properties is that you've assigned it a size, and throughout the lifetime of your game, its size won't change.

If you need to have an array that has a changing length, you'll need to use some other kind of array, like a List. However, I suppose you could also just make that element of the array null:

P1Deck[RandomPick] = null;

But this may cause other issues with your code (such as trying later to pick a card out of the deck that doesn't exist). You'll need to make sure that null cards in your code are ignored later. I'd probably lean to making a List.

I can't really find anything about Lists though... How can I declare a list of gameobjects instead of an array then?

See the MSDN docs.

Yes it does. Although the syntax is slightly different...

 List.<int> //Unityscript

vs

 List<int> //C#

Almost everything in the MSDN docs for .NET applies to Unityscript (and Boo for that matter).