Easy way to install programs in Ubuntu is using synaptic. If you are running Gnome in the menu System>Administration>Synaptic
type in the filter: system monitor , and ull get a list of things.
I believe Ubuntu have system monitor installed as default, it is in one of the menus as well, if you don't find it then in the terminal type:
Linux bring as default the program 'top' which you can just type in the terminal as well:
Conky is another nice system monitor and customizable.