*Calories in and calories out.
It is a maths problem, burn more than you take in.
Reducing calories and increasing activity worked.*
This. It is essentially the underpinning of every single type of diet out there.
There are equations to work out your BMR (basal metabolic rate - basically how much you burn solely at rest i.e., the bare minimum to keep your body going e.g. breathing etc.). In saying that, however, you'd be better taking measurements first thing in the morning, after you've been to the toilet but before you've ate or drank anything (stomach, hips, thighs in cms) and weight - note them all down. Then use myfitnesspal for tracking daily for one week. Be honest and log everything you eat & drink. Weigh out your foods if you can. At the end of the week re-take your measurements. If you've lost then your daily calories are already in a deficit. If you've gained, they're higher than you expend. You can work from this to lower them (by 500 cals max/week) until you find your sweet spot so to speak.
Once you've found your calorie deficit number I'd recommend building your meals around protein. Choose protein source first & then complement with carbs and/or fats. Protein keeps you fuller for much longer.
I like iifym style as I've no restrictions on what I eat. If a donut fits in to my macros for that day I eat it 😋
I also find meal prepping a day in advance helps me stay on track as I can track it all in MFP ahead of time & see what I'll be eating ahead of time. Can also tweak meals to meet my calories for the day.
Make sure you're also drinking plenty of water (2l per day minimum) as sometimes we feel hungry when we're thirsty.