Excellent that you are going to be taking better care of yourself !
Start by adding clean meals to your day. That means fruits, veggies, lean meats, legumes and beans. You will find that the better you eat, the less junk food will appeal to you. Don't drop a 'junk' food from your diet without adding a 'real' food to your diet.
It's not all about calories; 400 calories of lean meat and produce will do a lot of benefit to your body where 400 calories of cookies and fried foods will just make you sick.
Think health more than what you weigh, and think nutrition more than calories. And start getting in some exercise every day. Whatever you can tolerate, every day. Walk, do crunches, free weights....there's a great youtube channel called
fitnessblender that has different things you can do.
Above all, do not think of it as 'going on a diet', because that suggests that someday you'll go 'off the diet'. What this really is is treating your body with the respect it deserves, feeding it well, and keeping it strong. This, for me, is most difficult, as when I get to a weight that makes me happy I start to think that it's OK to eat some 'junk'.....see? When, really, junk food makes you sick regardless of how much you weigh!
So becoming more healthy is a daily, ongoing process. Take it on with happiness, and be kind to yourself along the way.
GREAT GOING