f you don't change your eating habits but you start exercising and moving around more you will lose weight, although it will take much longer.

Determine the amount of time you will dedicate to exercising.
Decide on your target weight.
Estimate the amount of weight you must lose the first week. Increase as you progress
Figure out your main form of exercise - walking, treadmill, running, etc.
Do this exercise every day around the same time.
Start out slowly, your sessions will get longer as your body adjusts to the exercise.
To prevent yourself from getting discouraged, give yourself a small reward at the end of each day.
Move around every chance you get - climb stairs instead of taking the elevator, park in farthest spot in the lot, clean house more (housework burns calories too, especially vacuuming and scrubbing bathroom)
Any time you're standing still, sway back and forth, tap your foot.
Go inside the fast food restaurant. Don't use the drive-through. Better yet, avoid fast food restaurants altogether. Fast food tends to be very high in calories and fat.
If the bank's open, go inside. Resist using the drive-up ATM.
Don't pay at the pump. Go inside the gas station.
Get a stability ball and bounce or just sit on it (making sure to keep your back straight and abs firm) while watching TV, working at the home computer, etc. Special desk chairs with "stability seats" are also available at a number of stores. Once comfortable with sitting and bouncing, add some toning exercises. These too become automatic and won't distract you from watching your favorite shows.

Set a target weight for a specific occasion, i.e. a friend's wedding in which you're a bridesmaid.
Drink lots of water. In addition to making fat more soluble, the water provides much needed sustenance to the muscles as they become accustomed to exercise. Choose water that is fortified with minerals like potassium,magnesium and calcium. These minerals, important to maintaining electrolytes in the blood, are depleted from sweating during exercise. They also aid in keeping blood pressure down which is important to any healthy person, especially one embarking on a serious exercise regimen.
Avoid caffeine and "diet pill" supplements containing caffeine. Although it will often decrease appetite and cause a greater initial weight loss, caffeine merely strips off water weight and dehydrates the body. If you are a coffee drinker, try to gradually reduce your intake or substitute half of your usual coffee grounds with chicory (French Market makes a coffee and chicory blend) to decrease the overall caffeine per cup.
Lift weights. Adding muscle to your body increases your metabolism which in turn helps your body's natural ability to burn fat.

Don't be too hard on yourself. You won't lose ten pounds the first week. Expect a gradual weight loss.
You will have soreness and muscle aches if you haven't vigorously exercised in awhile. Don't worry. The more you exercise the more you'll stretch your muscles and the pain will greatly decrease.
You can lose weight just by exercising only if you keep a moderate eating pattern- not a diet, per se, just eat sensibly three times a day with healthy snacks. No one can lose weight eating a whole bag of cheddar cheese Lays every day.
Watch for overall body condition changes, not just weight. Sometimes when making a major change to activity level, actual loss of weight is slower due to a gain in muscle tissue. Watch for changes in the fit of clothing and how you look in the mirror, not just the numbers on the scale.