If you get a good smart trainer. Start by just trying to free ride for 15 to 30 minutes on Zwift. Do that for a month at 3 times a week.
once you feel good at doing that.
go to the work outs section start the back to fitness work out plan. It’s a 12 week plan to get you back to fitness.
I am 67 and use Zwift, Sufferfest for hard work outs and Rouvy to enjoy out door riding.
when you get ready to ride outside join the local bike club and see what rides they have scheduled. I ride Zwift with my local club twice a week.