Drink water. Build up gradually over a period of weeks, but get to a point of drinking three liters or more each day--mostly in the morning and early afternoon, so you won't have to wake up during the night to pee.
Use small plates and small utensils to eat. This actually makes a real difference. Similarly, with each mouthful of food put down your fork/spoon while chewing; pick it/them up again after you've swallowed.
ASAP after dinner, try to get a walk of ~20min. Doesn't have to be terribly fast, just get it in. And also after dinner, strive for zero calories (water or seltzer only) the rest of the evening. If that's not realistic, fill up on salad-y stuff.
This is big: DO NOT EAT WHILE AT THE COMPUTER OR WATCHING TV. You want to be conscious of your eating.
Avoid diet soft drinks, which have been shown unhelpful in promoting weight loss. Avoid regular soft drinks, too!--tons of sugar/sweetener in there, which stimulates the appetite for more.
But notice that, otherwise, I'm not saying to skip/avoid any particular foods. Once you learn to satisfy your belly hunger, not your mouth hunger, you'll generally make good food choices. "Going on a diet" implies that at some point you'll be going *off* the diet; so instead of dieting, focus on behavioral changes.
Full disclosure: I lost ~60lbs a couple years back. Earlier this year, I got sick/stressed and got away from my good habits, and gained back about half of what I'd lost. I've gotten back to good habits, and the pounds are going again--slowly, maybe 1-2 pounds lost per week, but pretty steadily.
Good luck to you. You're smart to be working on losing the weight now, when you're still young. If you wait (as I did), your skin will lose its elasticity and, even when the weight's gone, you can look pretty saggy.