If you have only 30 minutes a day - RUN.
Running is the best exercise. Going back to anthropology, human beings were designed to hunt and gather. Hence, if you want to look and feel like a hunter - Run. Go gradual, slowly increase how much you run - but if you want to lose weight - there is nothing better than running.
Also - remember the 50-30-20 rule - 50 % of what your weight is depends on your diet. 30% on your cardio and 20% on your muscle training.