No preparation of any kind is necessary, but you must acquire at least one book:
James E.G. Zetzel, Cicero, De re publica: selections (Cambridge: Cambridge University Press, 1995).

The translation to buy is Niall Rudd's:
Cicero, The Republic and The Laws. Oxford World Classics series (Oxford: Oxford University Press, 1998)

There is also a nice new Oxford Classical Text that includes De re publica:
J.G.F. Powell, M. Tulli Ciceronis, De re publica, De legibus, Cato Maior De senectute, Laelius De amicitia (Oxford: Oxford University Press, 2006).

Make sure you have a good dictionary, at least a full as:
D.P. Simpson, Cassell's Latin Dictionary (New York: Wiley Publishing, 1968).

Zetzel's edition has an overview of the work, but you might also like to look at the following:
Paul MacKendrick, The Philosophical Books of Cicero (London: Duckworth, 1989), pp. 45-65.