SBCL is a well-established free Common Lisp implementation on which FEMLISP is developed and regularly tested.
On Debian/GNU Linux, SBCL can be installed using
sudo apt install sbcl sbcl-source sbcl-doc
If you do not have administrative rights for installing SBCL, you can also download a binary package for your architecture from the SBCL site (see SBCL Homepage). Installation of SBCL is rather easy: After you have downloaded the binary, you could simply create a symbolic link which links the file run-sbcl.sh to bin/sbcl (or wherever you put your binaries).
Although FEMLISP will probably work with many other CL implementations, the precise installation procedures can be quite different, and we do not want to handle this here.