If you don't eat well one meal, don't beat yourself up. And also don't decide that your diet is now 'broken' and there's no point going back. Instead, just eat well at the next meal.
Don't go too extreme. You'll be miserable if you go from full junk and soda to salad with no dressing. Instead, start with an exchange. An "eat this, not that" kind of thing. So, don't have soda, have flavoured sparkling water--or even diet soda (not great, but better than full fat). Don't have a candy bar, have a small square of dark chocolate.
Try to overhaul one meal at a day at least. So, if you're eating junk for breakfast, have half an avocado on a piece of wholemeal toast--it's delicious, it will fill you up, and it's very good for you. Or if you're having something very calorific for lunch, swap it out for a grilled chicken thigh and half a (small) jacket potato.
It's summer! The grill is your friend. Makes very boring food taste great. Grilled boneless skinless chicken thighs are amazing. You can make a bunch at once and use it for almost every meal. If you like fish, even better.
Exchange bigger plates for smaller plates and wait 15 minutes after a serving to see if you're really full or if you haven't realized it yet. Drink tonnes of water.
Go for a 15 minute walk every day. Don't count this against your calories--it's really to clear your head.
If you can, get some help. Whether that's an app, an accountability partner, or a dietitian: they will help you keep track.
Don't do fads, don't do extremes. The goal is 2lbs a week max. It doesn't seem like much at first. But it's also nearly four stone in six months. If you plateau, don't get mad at yourself. Just keep plugging along.
Be as kind to yourself as you can. Really and honestly: your worth doesn't depend on your size. Feeling good in your bones is the goal, not being worthwhile.