I am out of shape how much cardio should I do ,
If you have any concerns about your health it is a good idea for a doctor to give their OK before starting on an exercise program.
The safest thing to do is to use your heart rate to determine exercise intensity. I like the formula 214 - (age*0.7) for maximum HR for women. Your profile indicates your age is 23 so your MHR is estimated to be 198. Your target heart rate is 50-85% of your MHR which in your case would be 99-168 beats/minute.
Exercise in your target heart rate. Shoot for 20 minutes starting out but stop if you feel faint, dizzy, or short of breath. Conventional wisdom is that long workouts at the lower end of your target heart rate are better for burning fat and exericising at high intensities is better for aerobic conditioning. Lately some exercise physiologists have been challenging these notions though.
Consistancy is more important than anything else. Workout often and try to increase your time and/or intensity as your conditioning improves. Just don't go nuts and burn yourself out and quit.
If you don't have access to a heart rate monitor you can take your pulse for 6 seconds then multiply by 10 to see if you are in the ball park. If you count 10-17 beats in 10 seconds you are in your training zone.
start of lightly very lightly, do about 30 minutes of cardio 3 times a week, and then every week add more to it. If you are at least a bit in shape you can do 5 times a week. Remember to always give yourself a day rest.