Xbox 360 does not support stereoscopic 3D. There is an API to retrieve custom settings for "3D enabled" and "screen size" so you can determine how and how to best render 3D content (screen size can be used to determine default convergence and parallax), but not to enable mode 3D or (but you can do it manually).
For 3D rendering, you need to create two scaled half-resolution images and put them in one 2D frame, usually side by side or on top (or other less common formats, but it’s best to stick with these). The user must manually enable the 3D option on the TV and select the appropriate 3D format (SBS / OU) and the TV, and then separate the two images, zoom to fill the screen and display them separately for each eye.
If you have a simple 2D game, such as a side scroller, and you just want to add some simple depth layers, then this is pretty straightforward, adding a horizontal offset for each eye. Rendering stereoscopic 3D pads for other games is not easy. Look for Sony's PDF for an explanation of what works and what doesn't.