Hi Faye
I hope your day is going well. If you haven't already decided how many calories a day you're going to start with, one way to calculate it is to work out how many pounds you weigh now and how many pounds you'll weigh at your target weight. Then multiply the pounds you weigh now by 10 to get your starting calorie amount, and multiply your goal weight by 12 to get your maintenance calories. Once you've got settled into eating your initial calorie amount start reduce your calories very steadily (maybe 5% per fortnight or month) until you're at the maintenance calories figure. You can always step it up again if you struggle. You should find you'll lose weight gradually and your body will adapt to the reduced calorie intake without leaving you too hungry.
If you eat more one day don't worry - your daily calorie intake is a guide not a 'pass or fail' but don't under eat the next day to compensate or you'll end up yo-yoing.
Food that makes you happy is the healthiest food of all so eat things you really like!
Good luck and enjoy your challenge.