Firstly, don't see it as a diet as such, have a think about what you enjoy eating and try and think of either alternatives or ways of making these on plan.
If you have certain things you enjoy that are obviously not healthy etc but want to keep eating them try, for example a bar of chocolate of packet of crisps, check out the syns, if they're ridiculously high maybe try and find an alternative which will satisfy your craving. I always had crisps which were probably 6-8 syns and have changed them for a 4.5syn packet and have one a day.
Plenty of water.
Use your head. Free foods are free but if you want to lose weight you know there is a limit to in your diet where if you're taking in too many calories you won't be losing weight. Just because pasta is free you can't have a bucket for example. I know so many people who really smash the carbs and moan they never lose weight despite being on plan. Same goes with fizzy drinks, pepsi max is free but it's not going to help your weight loss if you're drinking 2l a day.
For me, the big things I did were cutting alcohol, milk, bread out of the diet and ensuring I packed veg in my meals.
The first month will be very trial and error but you will get an idea of what works for you and what doesn't. I was having omelette and beans a lot for example as it's free but realised quickly that changing this for a different lunch and the weight dropped quicker.
And don't feel guilty for using 15 syns a day. The first few days are easy to accidentally use 20 or so as you miss small things off. Fresh fruit juice, gravy, ketchup etc which you might not necessarily realise would be synned and you quickly find out they are! Also don't kick yourself when you gain weight in a week despite being on plan, it happens to all of us. Just carry on to the next week and aim to lose it. Try not to get on the scales every day and become obsessed with it, it's a horrible habit!
Good luck! Think of it as a lifestyle change rather than a diet, eat things you like, don't starve yourself and fill yourself with good stuff and you'll smash it!