How should I connect a new outlet cable to a box with two existing cables?
Assuming that one cable is the incoming power and one goes out to more outlets or lights, just keep everything connected by color--blacks together and whites together. 

I'm a little concerned that you're doing electrical work without the skills to trace a simple circuit, though. Please be sure that you have an understanding of proper connection techniques, wire size requirements, and other best practices. A little reading of a house wiring guide would be wise. You don't want to burn your house down.