I'm no Photoshop user, except in a very mundane sense, so this is a very straightforward question.

Imagine I have a PSD with a document size of 500x500 pixels.

It has one layer, upon which sits a green square 50 pixels on each size and sitting at (250, 300).

What I want from this document is a PNG, a 50 pixel green square.

How do I go about this?

I know this is simplistic -it's just an example. I essentially want a crop to the content of a layer. I'm using CS6.