An assessment would be critical before recommending a specific program. Generally speaking, however, for someone who hasn’t exercised in a long time and is obese, I frequently recommend swimming and aquatics classes to take strain off the joints while still offering a muscular and cardiovascular challenge.
I start all my clients with a set of foundation movements. This is done with all clients to varying degrees. Novice exercisers often are not ready to do much more than begin to move all of the joints. Experienced exercisers often have develop compensations or poor mechanical movement patterns. Even elite athletes are not immune to having fallen into poor mechanical movement patterns. Also, the movements are quite good at uncovering ROM issues, movement and strength imbalances, etc. Many times these movements are very challenging for sedentary and obese clients. And serve as the initial exercise program design.
As the others have mentioned already, it depends on the person and their general health assessment. However, if there are no major orthopedic issues or injuries, walking or swimming are both good options. Strength training and flexibility work are important as well, and should be a part of the fitness program.
Hiring a trainer is a great option for a few sessions to get on a program safely.
I would also start with walking. If you don’t have any issues with your knees or hips, then walking for a few minutes every day would be the safest way to start. You can go for 10, 20 or 30 minutes at a time and when you feel comfortable with that duration then keep increasing it every other week or so (depending on how you feel). Hiring a trainer even for a few sessions would be a very good idea as well.