Image by Sha Sha Chu
I have just been through these logic primers by Brian over at his fantastic blog Apologetics 315.
Looking back I really wish I had read these sooner as it would have helped me to clarify myself better and engage in discussion a little more easily.
It is an easy to read and not too long series of articles that take you through a basic understanding of logic. He covers such topics as how to form valid arguments coherently and how to avoid fallacies. You can access it here