Worked out off and on all my adult life. most of the time walked a lot at work. then switched to web job. how to get back in shape safely?

Just start: The first step in completing any hard task is to just start. Unless you have any pre-existing medical conditions or health concerns, start slowly & add volume and intensity in small increments. Mix in both cardio and resistance exercises. Shoot for at least 20 minutes/day and 3/week and build from there. Try different exercises to find what you enjoy. If there are health concerns, talk to your doc.