I highly recommend writing your own toy version of LISP in whatever language you already know. Just enough to get the basic eval loop working (+ macros if you feel brave).
It is fun and IMHO the best way to learn LISP.
If you get stuck read through one of the many examples online and then try again without looking at those examples.
This book is a great introduction to Common Lisp and Lisp in general. Very no-nonsense. Happy to see it here!
I just wish many of the software projects the code in the book depends on had been maintained. Portable Aserve and the Shoutcast server are two examples that come to mind off the top of my head, and of course the Lisp in a Box environment is long gone, and the replacements, like Portacle, are also getting left behind.