What bike are you getting?
Since this is your first bike get some inexpensive platform pedals. Once you get comfortable on the bike you can try clipless.
Get a new helmet. I replace mine every 3 years. Most folks I know do the same.
If you want to be seen get lights (headlight and taillight). Reflectors don't work.
You'll need a floor pump to pump up your tires before each ride.
You'll need a mini pump to the with you.
Get a seat pack to carry a spare tube, multitool, patch kit, tire levers, etc.
You'll need cycling gloves, cycling shoes (road or mtn), cycling shorts, jerseys.
If you'll leave the bike unattended when out riding you'll need a lock, cable or U-lock.
And the list will go on and on and on.......
