Hi.I need to lose 20 kgs in a month.I m 22 n height is 158 cms.I don't exercise much... I need a diet plan which can help me in achieving our goal.

Sorry, but NO realistically possible.

You'll need to drastically reduce your calorific intake and increase your aerobic activity.
(Less food more excersise)

I have 1 diet plan that I tell people with extreme goals but before I say ill always tell you...

Warning: This is dangerous may cause dehydration or cause you to pass out.

Ok here you are...

10 day crash diet drink only a pure fruit and vegetable drink that you yourself blended with fresh fruits/veggies. (eat NOTHING)

Next 5 days you will need to ease yourself into food start light with some protein than introduce more calories a day until you get to maybe 900 calories. (IF THAT) In this time introduce light jogs and easy things like situps or if you can pushups. (you will gain back some weight from your body gaining muscle/water weight)

Next 10 days pick it all up do weight training increase cardio 1 mi jogs daily

Last 5 days drop weight training and put it all in running and cardio.

In all you may loose 25 lbs.

I don't recommend this because of dangers and failure rate.