Pump and a air gauge, a helmet, and a water bottle. I also like to wear eye protection -- regular sunglasses or the cheapest clear safety glasses at Target/Walmart. Grab a bike multi-tool while you're there.
Beyond that just ride. Get the saddle height right and ride some more. When I started up again in my mid 40's I was amazed when I went 4 or 5 miles. I'm in my 60's now and 15 to 20 miles is just a ride after work and before dinner.