I want to install a system monitor which looks something like the one in the image.
It's a conky with Google Now theme.
You want to install
conky-manager and then download the theme.
Unpack the downloaded archive with theme into
~/.config (which is a hidden folder in your home folder - you can unhide it by pressing
ctrl+H in your file manager). Now the theme should show up in conky manager.
conky-manager is not in the elementary repos and needs to be installed from developer's PPA -