Whether you are new to the gym or frequent the gym quite often, one machine that surely gets a lot of use would be the treadmill. The treadmill is an excellent way to get your cardio in. One way of ramping up your cardio session is by doing intervals on the treadmill. Interval training has been associated with burning more calories.
Interval training is high-intensity training for a short period of time immediately followed by a period of less-intensity training. Say as an example of a beginner, you run for 30 seconds to 1 minute then follow that with walking for 1-2 minutes. Mix it up to get to your targeted heart rate and then bring your heart rate back down gradually.
Start out slow and go at the pace most comfortable for you until you are ready to advance. Consult your doctor before starting any new workout routines.