First, get lights and some reflective gear. Confidence is worthless if drivers can't see you. Then, start slow...get some riding in during non-traffic times, like early in the morning or the weekend and gradually get more adventurous. Then test tide your commute. Cars don't actually want to hit you, no matter how aggressive they are (well, there are some rare exceptions) so if you are careful and most importantly, PREDICTABLE, then cars will go around you with little drama.