The Nvidia GPU-based denoiser is available by simply adding an output channel with the same name as another but with a unique name (e.g. "_denoise" appended to the end) and the denoise checkbox ticked. It renders at the same time as the regular AOV of the same name.

Render Device

Information on getting started with GPU with Arnold can be found here.

If you have only one Nvidia-based GPU it should automatically select it. You may switch between GPU and CPU during live rendering, but it is recommended to start with GPU so that it can get the scene properly set up internally.  After that, switches should be fast and easy to compare the two. You may also manually select your GPU, and in so doing see its available memory: 

Render Device Fallback