Ask HN: How to License My Program

I have spent the last few months working on a program for a friend of mine who is an investment banker. His company uses an online service to track deals and allegedly provide meaningful statistics back for them, however the system does not correctly combine the data in a way useful to them. My program gets the raw data and combines it into what they would like to know. He mentioned recently that he knows many other people in their same situation that would benefit from my program, and encouraged me see if I can make any more money off of it. Knowing little about the business side of programming, my questions are: 1) Should I license my program in order to protect myself? 2) What license should I use? (There is no disagreement on ownership, as my friend recognizes the work as my intellectual property (Should I get this in writing? (I wonder if this guy knows lisp...))) Thank you for any advice or wisdom you can give.