Don't rush it, just do it. You would push yourself till you were a bit puffed, had a rest, then did some more. Start gently for a the first week or more and gradually you will find exercise is becoming easier. If the weather is bad, drive to a large shopping centre and walk there. As you improve you might even like to try a few steps on the stairs or escalators.
Some people buy or hire a treadmill or an exercise bike.
Whatever you do, make sure you enjoy it. Do it with your wife or husband, your children or grandchildren or your dog. Drive to the beach or a park and walk there. Getting back into exercise can give you the confidence, fitness and strength to go to your favourite sport, or the movies or a theatre show. Why not try a swing or two of the rusty golf clubs, or take out the dusty bowls.