Firstly I'd say don't try and do it all in one go. Make a few changes, then once they've become a habit make a few more. Stuff like less sugar in/on tea, cereals or whatever else, trade other drinks for water.
Secondly, how big are your main meals? It sounds counter intuitive to eat more when trying to eat less but if your meals aren't filling you up then you're more likely to get hungry and give in to cravings. Everyone is different, for some people the grazing way works well to lose weight but I find that when I let myself snack, I reach for the biscuits, crisps and cake bars and end up eating way more sugar. Aim for things that are high fibre and low GI and don't be afraid of a little fat (obviously you don't want to fry every meal, but there are 'healthy' fats that help us feel full and satisfied.
If you like junk food then try alternatives and 'fake-aways' where you can recreate your favourite foods but you know how much sugar, salt etc is in them. It honestly is a tricky one, I haven't worked out how to stay off the sweet stuff myself yet! Chocolate is my weakness. Also, look for the lowest calorie, lowest sugar version of foods. I love biscuits and we often have rich teas and chocolate digestives in the cupboard. If I went for 2 chocolate digestives I would be having about 165 cal. and 10g of sugar, if I went for the 2 rich teas it'd be more like 70 cal. and 3 g sugar. I still get my biscuit fix, but with a lot less sugar.
Try aiming for 75% healthy, 25% junk for now. 25% junk food might sound like a lot but if you think 'oh I'm eating a lot of junk food, I can eat better than this' then those foods won't feel off limits and you might find ou crave them less