Step 1: Don't take advice from anyone who confuses "lose" and "loose". If they can't write the correct word, chances are they haven't read much about it either.
Some basic facts and advice:
Muscle burns fat. The more muscle you have, the easier it is to KEEP the weight off once you've lost it.
Muscle pulls glucose (blood sugar) out of your bloodstream, and can then either use it for energy immediately if there's a need, or store it as glycogen for when you are exerting the muscle to a high degree.
Fat calls also pull glucose from the bloodstream when your blood sugar levels are high (like after a meal) and stores it as.... fat.
If you work your muscles hard enough to make them use their stored glycogen for energy, they'll be forced to replenish their stores, and will *compete* with the fat cells for the glucose in your blood.
See where this is going?
If you do high intensity cardio, or High Intensity Interval Training (HIIT), or lift fairly heavy weights, you'll both burn calories *and* change where and how those calories are stored. Doesn't have to be really heavy weights, but it can't be those little pink dumbbells either.
When you restrict calories (diet), you lose muscle, unless you are also exercising enough to give your body a growth signal to balance things. Muscles use energy even when you're just sitting on your butt at a desk. When your calorie intake drops, your body tries to compensate not just by using fat stores for energy, but also by tearing down muscle. It tries to adapt to the "new" diet. Problem is, when you come off that diet, you have less muscle, and so then your calorie needs are lower than they were before the diet. If you eat the same as before the diet, you regain the weight you lost, and then some. It's the classic yo-yo effect
Unless you've built up a bit of extra muscle, that is.
So diet and cardio are good, but diet, cardio *and* moving some heavy iron around at the gym (or doing HIIT style cardio) is the best combo for weight loss.
Don't try to lose weight too fast. A 500 calorie a day reduction is plenty (unless you're morbidly obese) and will limit the amount of muscle you lose (and even let you build a little while losing). And if you're not going to the gym every day, remember you'll have to either eat more on gym days or less on non-gym days so you don't eat too little to meet your body's demands. A 500 calorie a day reduction will toe the line and won't send a really strong signal to your body to start eating it's own muscle, but if you goof and eat like you weren't training on a training day, you just might wind up at a 1000 or 1500 calorie deficit and lose muscle instead of fat.
So in a nutshell? Yes, light (ok, medium) lifting will definitely help without adding bulk.