If you just want a Lisp environment, go ahead and grab Lispbox, an Emacs bundle pre-configured to work as a LISP IDE. Download the appropriate version from the official site.
If you want to really explore what Lisp is capable of on a regular
basis, get yourself a copy of Emacs. It’s a really great environment
and, if you put in effort, it will help you out a lot over the course
of your career. (Things like AucTeX, Magit, and multiple-cursors
are indispensable tools.)