Name the file init.lsp and place it in .../maxima-x.x/init.lsp.
The file can only contain lisp code. My maxima wasn't able to find
it if it was placed anywhere else.
Two examples of things you can do with init.lsp:
To autoload a function "hermite" located in the file specfun.o, use
(defprop |$hermite| #"specfun.o" autoload)
(add2lnc '|$hermite| $props)
To load the file "solve_patch.lisp", use
(load "solve_patch.lisp")
Barton