Questions about API and Functions

I'm starting to build some custom modules for whmcs.

Actually my need is a module that changes a password of a custom system, when the cpanel password is changed in whmcs.

I'm trying to find where i can attach my function to work when a administrator change cpanel pass via whmcs.

Also would be cool to find a function to do the same, but with the client area password. ,

I already searched on docs and forum and didn't find any info.

Also would be great a function(while) to fetch all client data from whmcs, and data for cpanel.

I appreciate any kind of help and expect to release some free modules in setember.