What do you already have?
Standard things in no particular order include:
-Helmet
-Sunglasses
-Roadside repair kit with spare tube(s), tire levers and some method of inflation (CO2 cartridges or pump) and a way to carry it
-Water bottles and bottle cages on bike
-Clipless pedals and matching shoes/cleats
-Handlebar computer for speedometer or advanced functions (including HR and or power for folks really into training and or racing)
- Bike shorts with good chamois
- Bike jerseys for different weather conditions with rear pockets to hold food, wallet, etc.
- Bike rain jacket that stuffs into pocket for rides on threatening days
- Bike gloves
-Floor pump at home for easy pre-ride tire inflation
-Basic tools for home repairs and bike maintenance
-Work stand for bike if you do a lot of home maintenance
The list could keep going as there are lots of accessories folks use. But the basics are related to: safety (helmet, gloves), self sufficiency on the bike (repair kit, water, food), comfort (bike shorts, good jerseys, rain gear) or ongoing bike maintenance (tools, lubricants, cleaning products).
-Dave