Lights, helmet, tools & tube, water bottle, rearview mirror, and a big bottle of chain lube. Use the lights day and night. They are mainly to increase your visibility. Clothes should be non-cotton moisture wicking, not black or camo. Bright and colorful may be dorky, but the helmet already has you in that zone.
As for carrying, I started with a messenger bag with a long enough strap that I could sling it around back, at butt level. I did a backpack for about a week. Too hot and sweaty on the back. Finally gravitated to a rear rack. Started by bungie strapping the MB or BP to it and then got the pann...bike bags. I ride in Houston, so heat and humidity are a key element and since its flat here, weight is not a huge issue. Keep things like that in mind when you read the advice here. Everyone's situation is unique. I know my highest service is as a bad example.