Congratulations on starting a healthy weight loss program. As I've said before, the only way to lose weight and keep it off is to adopt a lifestyle of eating less and exercising more. Of course, I know this is much easier said than done, but if you burn more calories than you eat you will lose weight and keep it off.
For effective weight loss, I think it is best to work out on an empty stomach. Whether it be aerobic exercise or lifting weights, exercising before you eat burns fat much more effectively than when done after a meal. For many people, the best time to exercise is first thing in the morning because an overnight fast causes blood sugar levels and carbohydrate stores to be at their lowest. Rigorous walking or weight training when you wake up causes the body to access fat for energy much easier and faster to meet the caloric demands of your workout than later in the day. If you just can't get up for a morning workout, at least try not to exercise until two to three hours after eating.
I don't recommend eating before a workout if you are trying to lose weight. This is a common mistake and usually is one reason why people just can't seem to drop pounds despite expending lots of effort in the gym. Eating before a workout actually supplies the body with energy, which it uses during exercise, especially if you ingest high glycemic-index carbohydrates that rapidly turn into blood sugar. Taking in carbohydrates before a workout causes the body to use sugar as fuel, preventing the burning of fat calories and keeping fat stores locked away. If you want to trim excess body fat, cut out carbs before you exercise and work out on an empty stomach.
Andrew Weil, M.D.