First of all, you should check to make sure that the package isn't already available. For example, typing M-x apropos <RET> python <RET> lists all functions and variables containing the string ‘python’.
It is also possible that the package is on your system, but has not been loaded. To see which packages are available for loading, look through your computer's lisp directory (see File-name conventions). The Lisp source to most packages contains a short description of how they should be loaded, invoked, and configured—so before you use or modify a Lisp package, see if the author has provided any hints in the source code.
The command C-h p (
finder-by-keyword) allows you to browse
the constituent Emacs packages.
For advice on how to find extra packages that are not part of Emacs, see Packages that do not come with Emacs.