Helmet and lights for safety, spare tube(s) and basic tools for flat on-road repair. Padded gloves, cycling shorts (or at least clothing without abrasive stitching/folds in the seat area). Also think about getting clip-on rearview mirror.
When riding, make sure vehicle drivers can SEE you - and don't take risks. Don't stop next to a vehicle - be behind or in front, so you can be seen. Pay attention to traffic - it is easy to get 'caught-up' in riding and not see dangerous situations.