There are several free versions of LISP available and you may use any that you prefer. Franz Inc. is a widely used provider of LISP and LISP based software and they provide a free version of their LISP at their download site. Franz has a great deal of relevant information at their site, particularly at their "new to LISP" site where you will find several useful texts. One of those texts is Stu Shapiro's "Common LISP: An Interactive Approach" which is available as a free download. This book gives a good sense of what it is like to work in a LISP interactive development environment (IDE), though it doesn't deal with the details of any particular IDE. Paul Grahm's own site also has a number of interesting articles and other resources including a free download of his more advanced text ON LISP which is a follow up to ANSI Common LISP with an emphasis on how to use macros to develop languages on top of languages. (This book is a gem that was selling on Amazon for up to $300/copy before Grahm made it abailable on his site for free.)
Another widely used free version of Lisp is provided by LispWorks.
LispWorks and Franz are commercial providers. Their free versions are missing some of the advanced features of their commercial versions but those differences won't be important for you in this class. What you might want to consider regarding these two systems is their time limits. Franz gives a temporary license that has to be renewed every two months (at this time). LispWorks gives an unlimited license but it only lets you work for five hours at a time and will crash after five hours without saving anything you are working on. But it does give you a warning after four hours so you can save your work and restart the system. ANSII Common Lisp is standardized across all platforms so the code you write should work on any consistent platform, even the more bare bones free systems. The differences will be mostly in the IDE, including things like how their debugging tools work and how they support the development of stand alone applications.
If you are feeling adventurous you can look up "lisp" on sourceforge.net, go to
common-lisp.net, or just google free lisp.