Are you a morning, lunchtime, or evening exerciser? Time of day and how you fuel is important to maximize your workout effectiveness.
Eating should be done to correlate to the body's needs for the workout. One common misunderstanding is going to the gym to workout before breakfast. The idea is that if there is no sugar (glucose) in the blood then the body will burn fat. Unfortunately, it doesn't work that way.
When the body doesn't have enough blood sugar it will convert amino acids (proteins) into glucose (sugar). This is done by the liver and is called gluconeogenesis. There are 2 available sources of proteins to use-immune cells (immunoglobulins) and muscle cells. You don't want your body converting your immune system to sugar OR your muscles to break down for fuel every time you work out.
The best time to eat is before your workout. Not right before it- you definitely don't want to up-chuck on the gym's equipment- about an hour is good. The best choice of foods are low-glycemic carbs, lean proteins, and don't be afraid of good quality fats. This can be accomplished with a high quality meal replacement shake if time is tight. Otherwise, I can share with you some relatively fast and easy recipes.
And, if your workout is going to last more than about 45 minutes then you want to stop near the half-way point and take in a small amount of shake or a nutrition bar and a strong anti-oxidant/vitamin.
I prefer USANA Health Sciences Nutrimeal for my shakes and their Peanut Butter Crunch nutrition bar. There are a few other but you have to be careful about the types of sugars- you don't want to spike your blood sugar either.
One lady I know spends 2-3 hours per day at the gym and yet she just can't seem to lose the extra weight. Popular thinking says "calorie in, calorie out" so she must be eating too much of the wrong stuff. In fact, she eats very well but doesn't eat enough to sustain her during the workouts so she is not generating lean muscle mass. It is lean muscle mass that is responsible for burning "calories" and weight loss, lowering triglycerides, cholesterol, and improving health.