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.
Download Portacle - All-In-One Common Lisp
Otherwise, Common Lisp comes in many different flavors, or implementations.
Two popular open source ones are Steel Bank Common Lisp (SBCL) and Clozure Common Lisp (CCL):
Look here for other Common Lisp compilers, including powerful commercial implementations.
You can also try Common Lisp online:
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:
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.
You can also watch this video tutorial:
A good way to learn Lisp is to go through Practical Common Lisp book.
Read our documentation for more options.
You may want check out the Common Lisp Cookbook for getting started on Common Lisp ecosystem.
Check out this list of Lisp libraries.
The easiest way to download and load libraries is via Quicklisp. Follow these instructions to get started.
Here's a list of alternative guides for getting started: