NYMUSC is right on track.
Best time to do cardio is first thing in the morning, when growth hormone is highest, and blood sugar is lowest, as well as insulin is lowest.
Eat about 40grams of protein and maybe of bit of banana, or something that will keep you thinking straight (avoid hypoglycemia), and do your 30 minutes of cardio in the morning.
Interval training is most effective. I.e., don't train like a moron, doing countless hours of cardio. Go in, get your heart rate up, and repeat twice. Go home, eat.
Remember: resistance training is the ONLY training that increases your lean muscle mass (and hence your metabolism), and strengthens your bones, AS YOU GROW OLDER.
The more MUSCLE you have, the easier it will be to stay lean as every pound of muscle burns calories just setting there. Being anoxericly thin is an idiotic notion: it slows your metabolism to a crawl.
Don't pay a bit of attention to BMI (Body Mass Index). It's severely flawed for fit and muscular people.