As MonoVCPHG wrote, do lots of reading about polyamory.
After that, the next step is probably to decide if you're going to try opening the relationship or not. Know ahead of time that you will face fear and jealousy. The decision you have to make is to decide to try to overcome the jealousy. It can be overcome.
Today in an old issue of Loving More magazine I read an article about jealousy. The author, who is a counselor suggested using the phobia model as one of two methods that she recommends to overcome jealousy. It is basically a method of slowly and gradually being exposed to what triggers fear in you. Over time the triggers should have less and less of an impact on you.
Before working at overcoming jealousy, communicate all of your feelings to your partner. I can not stress the importance of open communication enough. In fact, I'm still working on this myself.