Hmm. It really depends what sort of development you want to get into. For a start
you should really read and have a play with all the information in "Unix in a Nutshell" (or "Linux in a Nutshell", if you are more interested in Linux than a more "generic" Unix) from O'Reilly.
Then, do you plan to do systems work (using C, perl, shell scripts etc.), application programming in any other language (
Java for example), database development, X-Window GUI development, server/network development or whatever? All of these areas have different knowledge requirements.
Can you give us a few more details about what you want to do?