Building on the other replies, sequence and class UML diagrams are particularly useful and easy to learn. Read Fowlers short book on the subject (forget the name of the top of my head). You can be up and running in no time. Plus it has the added benefit of familiarizing you with those two diagrams, which are widely used in the software engineering literature.

It's easy to go overboard with UML. As Fowler says, it's great for sketching, but don't try to use it for much more than that - I've gone down that road and wasted a lot of time. But I use sequence and class diagrams quite a bit to do exactly what you need.