Walk. Gentle exercise to start. Get good shoes. Do things that you enjoy.
Stretch after exercise. Get into that habit. Really stretch - start with calf stretching particularly, and do some yoga or Pilates to assist developing your joint mobility.
If you've never done much exercise, you won't have the muscle memory which helps your body move efficiently, so you'll need to give yourself time to develop that.
If you really don't do anything at the moment, I'd recommend a personal trainer for a few months, to get you started, keep an eye on your form & technique, and give you ideas for trying stuff you might enjoy. Also, if you're paying a fair bit of money (PTs in my area are around £35 a session) for a once a week or once a fortnight appointment, it gives you a structure.
People say they don't have time: try a week of making time for an hour long walk every day. Once you squeeze it into your diary, it soon becomes routine. It's amazing how you do have time1
I'll be 60 soon & I'm the fittest I've been since my 20s (and I've recently dropped two dress sizes) - I run, I weightlift a bit + Body Pump, I do yoga/Pilates, and I do several dance classes a week. I want to live a lot longer and this is how I'm doing it.