Keep myself from gaining weight by exercising?

Yes, it helps!: First, weight per se is not the holy grail. Leaness is. Exercise helps prevent weight gain, but increases your lean body mass. You will look more toned. Apart from caloric burn from your sporting activities, increasing muscle burns more calories at rest! finally, some people do not lose their "desired weight." but numerous studies (cooper clinic), show that exercising trumps being overweight.

Watch your intake: To be worthwhile and effective, exercise should be viewed as fun. There are more benefits than weight control received from adequate exercise. Remember that 100 calories consumed extra a day will become 10 lbs in a year on your body so avoid anything in a wrapper (does more damage to your body than weight gain), add in walking more, find some fun activities, and remember balance :).

Don't weigh yourself: Lean muscle weighs more than fat so you may initially weigh more as your muscle builds. That muscle will burn more calories at rest (bonus! more calories burned when not exercising) and will slowly burn away the fat. Focus more on how your clothes fit. And realize that 100 extra calories a day will add to 10 lbs a year so avoid those small extras you consume that will negate those workouts.