(require "asdf").
Check that you have a recent version using (asdf:asdf-version).
For more details, or if any of the above fails, see Loading ASDF.
(asdf:load-system "my-system").
See Using ASDF.
/,
again in a location where ASDF can find it.
All else being equal, the easiest location is probably
~/common-lisp/my-system/.
See Configuring ASDF to find your systems.
(asdf:load-system "my-system")
to make sure it's all working properly. See Using ASDF.