Emacs ships with a lisp-mode for Common Lisp that supports basic stuff like indentation, S-expression navigation and font-locking.
SLIME, the Superior Lisp Interaction Mode for Emacs
If you're doing serious Common Lisp development you should have a look at the complementary SLIME, which gives you the ability to develop Lisp programs interactively. Quicklink: see Portacle, the portable and multiplatform development environment for Common Lisp. It's the easiest way to get SBCL + Emacs + Slime + Quicklisp + Git.