If you feel you must do cardio and weight training on the same day, you really have to eat for it or you will hurt youself. Depending on your goals, strength and muscle vs cardiovascular endurance...put the one that is most important to you first.
I generally suggest that you separate weights from cardio by 8 hours or so if training them on the same day.
My
personal preference though is to train weights and cardio on different days. For example M, W, F...weights. T, Th...cardio. S/S...off. Too bad my schedule won't allow this. The best I can manage is weights EOD, Cardio every 4th day and off every 4th day.