Walking is wonderful for losing weight, anything to get you moving is the first step to success!

Later you might want to add weight/strenght training, just work at your own comfortable pace.

It is currently recommended that we are physically active every day, exercising aerobically three to five times a week (e.g. brisk walking, cycling, swimming). However, begin with exercising aerobically three times a week. If you have led a sedentary lifestyle until now, leave a day or two in between your exercise days.

How long?
The most widely accepted minimum length of time you should spend exercising is 20 minutes (which does not include the warm-up and cool-down). The maximum is one hour, depending on the exercise you choose. If you are a beginner, try starting with ten minutes. It's better than nothing.

Good Luck, I love exercising so much now, that I dread having a day off.
Take care!