To download LTspice, visit http://www.linear.com/designtools/software/#LTspice
To download NGSpice, use the Ubuntu Software Center and search for NGSpice.
Either one of these programs is sufficient for the class. I use LTspice on Ubuntu 14.04 and I use LTspice on Windows 7 and 8, and Mac OS X.
To make good documentation for a simulation, it helps to have a schematic. While pencil and paper is good, it is even better to use a computer. LTspice includes schematic entry. For NGSpice, KiCad can be used. The class will cover the KiCad integration with NGSpice.