Hi there =)
I think that aiming to eat a little less, drink more water, and get in extra activity is a very healthy-minded thing to do.
You have those ideas in mind, so when you say you don't know where to begin, perhaps you can pick one of those three things and put your energies there to start.
Drink more water... that's easy enough, right? For a week or two simply focus on replacing your usual drinks with water. =)
Moving more... that actually can be easy enough, too! Stairs instead of the elevator. Walking once around a building before entering it. Aiming to take a stroll around your neighborhood each afternoon. Or finding a sport or activity you've always wanted to try, and trying it out.
Eating less... this is different for everyone, but for you I'd suggest quite literally eating less. For example, at dinner, aim to leave a little something leftover. Maybe even cut the meal in half or only eat 2/3rds (Please do eat the veggies and fruit, though! They aren't as many calories, and pack a lot of nutritional punch.) Or when you catch yourself going for a snack when you're bored, tell yourself that if you're truly hungry you can have something in 20 minutes. If 20 minutes go by and you still want something, fine, but sometimes after 20 minutes and a glass of water... you find you aren't really as hungry as you were before. It wasn't true hunger.
Those ideas are just to start. MOST importantly, keep in mind that you aren't aiming for perfection. You're just aiming to be a little healthier each day. So if you do eat something you didn't want, or happen to have a drink you didn't need, don't needlessly beat yourself up or give up. Just let it go, and do better at the next meal.
You can do this!
Best of luck!