Exercise and healthy eating are the key.
Exercise - try to get moving, not just through formal exercise like walking/running/swimming/lifting weights but taking the stairs, parking far away from your destination, whatever you can do to get into motion. Excess weight is a matter of more calories consumed than burned, so the more calories you can burn in everyday life the better.
Healthy eating - really cut back on processed foods, fruit juices, sodas, candy, fast food, simple carbs like pasta, white bread, potatoes. Eat lots of fruits and veggies, whole grains, whole wheat bread and whole wheat pasta in moderation, beans, lean meats (skinless chicken breasts, fish, ground turkey), and so forth. You can watch what you eat, even while you're breastfeeding - Weight Watchers for example gives breastfeeding moms extra calories while still setting overall calorie limits. Don't go on any fad diets that you can't live with in the long run.
Dancinggirl, it is too soon to be discouraged. 3 weeks is way too early to start seeing results from exercise; you need to stick with it for a good 12 weeks or so to start seeing some real results. Also, get a tape measure and take some measurements - upper arm, waist, butt, thighs - and compare measurements, not pounds on the scale. Particularly if you are lifting weights and building muscle, you can weigh the same because a pound of muscle weighs more than a pound of fat, but look very different in the mirror and have smaller measurements.
I think weight lifting is awesome, because once you put on more muscle that muscle mass burns more calories in a resting state than does fat, so it helps you lose weight faster. You don't have to have a fully equipped gym or join a gym to lift, either; a couple sets of small hand weights and a good tape like Joyce Vedral's can put you on the right track.
I of course need to practice what I preach; I have a healthy diet but it's hard getting time to exercise with a baby!! We did do an hour and fifteen minute walk today, so that's something at least.
There are a couple of weight loss and exercise support threads kicking around; check them out!
Edited to add: don't check your weight more than once a week, if that. Look at how your clothes fit, your energy levels, etc. for motivation. A healthy level of weight loss is a pound a week, don't set yourself up for failure by looking at the scale every day!