If you are a newbie or you want to get started as fast as possible, then Portacle is probably your best option. Portacle is a multiplatform, complete IDE for Common Lisp. It includes Emacs, SBCL, Git, Quicklisp, all configured and ready to use.
This is a JSCL Lisp REPL running on the Web. Note that it is not a complete Common Lisp implementation, but enough to have a taste of it:
Setup your environment
A Lisp setup consists of three parts: a running Lisp instance, a text editor connected to that running Lisp instance, and project setup and libraries.
The most common way is to use Emacs and SLIME as text editor/IDE and ASDF + Quicklisp for project setup and libraries.
SLIME is an extension to the Emacs text editor that connects the editor to the running Lisp image (called *inferior-lisp*) and interacts with it. It provides lisp code evaluation, compilation, and macroexpansion, online documentation, code navigation, objects inspection, debugger, and much much more.
ASDF is the Lisp version of Make. It is used to define projects (called systems), its dependencies, and load and compile the project.
Quicklisp is a library manager for Common Lisp. Use it to download, install, and load any of over 1,500 libraries with a few simple commands.
For an introduction on how to set them all up, you can have a look at this guide.
"Programs must be written for people to read, and only incidentally for machines to execute."
- Abelson & Sussman, SICP, preface to the first edition
"That language is an instrument of human reason, and not merely a medium for the expression of thought, is a truth generally admitted."
- George Boole, quoted in Iverson's Turing Award Lecture
"One of the most important and fascinating of all computer languages is Lisp (standing for "List Processing"), which was invented by John McCarthy around the time Algol was invented."
- Douglas Hofstadter, Godel, Escher, Bach
"Lisp is a programmable programming language."
- John Foderaro, CACM, September 1991
"Lisp isn't a language, it's a building material."
- Alan Kay
"Any sufficiently complicated C or Fortran program contains an ad hoc informally-specified bug-ridden slow implementation of half of Common Lisp."
- Philip Greenspun (Greenspun's Tenth Rule of Programming)
"Lisp is worth learning for the profound enlightenment experience you will have when you finally get it; that experience will make you a better programmer for the rest of your days, even if you never actually use Lisp itself a lot."
- Eric Raymond, "How to Become a Hacker"
"Lisp is a programmer amplifier."
- Martin Rodgers
"Common Lisp, a happy amalgam of the features of previous Lisps."
- Winston & Horn, Lisp
"Lisp doesn't look any deader than usual to me."
- David Thornley
"SQL, Lisp, and Haskell are the only programming languages that I've seen where one spends more time thinking than typing."
- Philip Greenspun
"Don't worry about what anybody else is going to do. The best way to predict the future is to invent it."
- Alan Kay
"The greatest single programming language ever designed."
- Alan Kay, on Lisp
"I object to doing things that computers can do."
- Olin Shivers
"Lisp is a language for doing what you've been told is impossible."
- Kent Pitman
"Lisp is the red pill."
- John Fraser
"Within a couple weeks of learning Lisp I found programming in any other language unbearably constraining."
- Paul Graham
"Programming in Lisp is like playing with the primordial forces of the universe. It feels like lightning between your fingertips. No other language even feels close."
- Glenn Ehrlich
"A Lisp programmer knows the value of everything, but the cost of nothing."
- Alan Perlis
"Lisp is the most sophisticated programming language I know. It is literally decades ahead of the competition ... it is not possible (as far as I know) to actually use Lisp seriously before reaching the point of no return."
- Christian Lynbech, Road to Lisp
"[Lisp] has assisted a number of our most gifted fellow humans in thinking previously impossible thoughts."
- Edsger Dijkstra, CACM, 15:10
"The limits of my language are the limits of my world."
- Ludwig Wittgenstein, Tractatus Logico-Philosophicus 5.6, 1918