Thirty minutes a day is a great starting goal if you are currently sedentary and overweight. You don't have to do all thirty minutes at one time - you can split it up into three portions of 10 minutes if you like. In fact, some studies show that this is more beneficial for your heart, bones and overall health.
The CDC in America recommends that adults spend at least 150 minutes exercising each week (roughly half an hour a day over five days), AS WELL AS muscle strengthening exercises on two or more days a week. These muscle strengthening exercises can be weight lifting, body weight exercises or even heavy gardening; whichever you prefer. For even greater benefits, the CDC recommends lifting the minimum to 300 minutes of moderate-intensity exercise per week + 2 or more days of muscle strengthening exercises.
As time goes on and you find more activities that you enjoy, you can slowly bump up your exercise to about an hour a day. If you can achieve that, then you're much more likely to be able to maintain a healthy weight into the future.
 CDC Physical Exercise for Everyone www.cdc.gov