Your best bet is to find some sites teaching how to use BSD Unix or Linux* shell commands and go from there. OSX does sometimes need a few open source extras installing to get the most out of it (I use MacPorts for that myself, but other repositories are available), but otherwise there's plenty of UNIX or command shell documentation out on the Internet to get you going once you've got the basic concepts.
*OSX is much much closer to BSD, but there's much more on the web to teach you Linux.