Settings -> Graphics: If it shows something ‘generic’ like ‘NV136’, then it’s not using NVIDIA drivers.
Go to ‘Software & Updates’ -> ‘Additional Drivers’. Select a recent/recommended one – I use ‘nvidia-driver-410’ at the time of writing (Mar 2019).
Let it update then reboot. If it’s worked, the command ‘nvidia-smi’ should show GPU status information.
NOTE: If it doesn’t work (i.e. comes up in low-resolution mode, and the ‘Settings->Graphics’ still shows ‘NV136’ or ‘llvm…’, then it’s not worked. IF THAT HAPPENS, the most likely fix is to DISABLE SECURE BOOT in the BIOS – secure boot stops some drivers being loaded. This was the problem on my machine, and disabling secure boot made the driver I’d already installed work.
Following CUDA installation instructions on NVIDIA site: Use Ubuntu package manager version
sudo apt-get install cuda
CUDA toolkit seems to be separate?
sudo apt install nvidia-cuda-toolkit
As per the CUDA post-install instructions – add the NVIDIA dir to PATH, install samples, compile them, run them.