Checklist (before you leave the house)

Cycling shorts
Tights or knee warmers (to go over shorts, recommended for winter riding)
Arm warmers
Ear warmers, headband, or cap for under helmet
Vest/Jacket (warm jacket recommended for winter)
Undershirt (long sleeve undershirt recommended for winter)
Jersey (you might want a long sleeve jersey in the winter)
Booties (shoe/toe covers, recommended for winter riding)
Gloves (should be warm for winter, consider glove liners as well)
Helmet (required on all rides)
Shoes
Handkerchief
Socks (wool recommended for winter)
Sunglasses
2 water bottles or Camel Back Hydration System
Heart rate monitor (optional)
Chamois butter (aka butt butter)
Electrolyte supplements (Cytomax, Accelerade, NUUN, or Powdered Gatorade)
Power bars or gel packs to eat while you ride
Food for after the ride (Recovery Drinks)
Floor pump (if you have one - keep in car)
Spare tire (if you have one - keep in car)
2 spare tubes (in your bike pack, correct size and valve type)
Tool kit (multi tool, tire irons, patch kit, CO2 and spoke wrench)
Cell phone (if you have one)
Towels (for after you ride - keep in car)
Clothes to change into after you ride