To run SBCL type sbcl at the command line.
You should end up in the toplevel REPL (read, eval, print -loop), where you can interact with SBCL by typing expressions.
$ sbcl This is SBCL 0.8.13.60, an implementation of ANSI Common Lisp. More information about SBCL is available at <http://www.sbcl.org/>. SBCL is free software, provided as is, with absolutely no warranty. It is mostly in the public domain; some portions are provided under BSD-style licenses. See the CREDITS and COPYING files in the distribution for more information. * (+ 2 2) 4 * (quit) $
See also Command Line Options and Stopping SBCL.