alright, i'll make this as intelligible as possible. as someone who's researched this extensively for years, i can tell you a few things that i know for sure (no personal assumptions, nothing that hasn't been confirmed by several trustworthy sources, no diet-culture myths):
1) eating less than 2,500 calories, as user Gina871 suggested, may not make you lose weight---hell, if i had 2.5k every day, i'd gain kilos within weeks!! it's generally recommended, in order to lose a pound of fat a week, to eat 500 cals less than your TDEE every day.
2) your TDEE is your total daily energy expenditure: the addition of the calories you burn a day through exercise, fidgeting, etc... and your BMR, or basal metabolic rate. BMR is the calories your body burns at rest (through breathing, heartbeat, brain activity...); essentially, the calories you would burn from lying still in bed all day. online TDEE calculators exist, but it is unique to everyone. it depends on gender, how active you are, your weight... but also your body fat percentage, and of course something no calculator can account for: genetics and individual variety. for example, i'm quite thin and short, but active: my BMR is around 1,300 calories and my TDEE ~300 calories above that. keep in mind this is much lower than the average individual!!
3) you really shouldn't restrict your caloric intake too much. the body adapts to a too-low caloric intake by reducing BMR (by decreasing heart rate and body temperature, for example), slowing weight loss. keeping this in mind, "starvation mode," or weight gain due to metabolic dysfunction from excessively low-calorie dieting, does not exist, no matter how much you may hear it. the true danger of very low caloric intake (aside, of course, from health risks) is that your body tries to make up for being starved by pushing you to crave and consume many calorie-dense foods, undoing progress. the moral of the story is not to starve yourself, and to keep viewing food as something necessary and pleasurable. an unhealthy relationship with food can ruin your life and the life of those around you :/ please be careful and be safe.
4) you can also raise your metabolism short-term!! drinks containing caffeine are effective for that, a few cups a day can make a few dozen calories' difference. i recommend green tea (the teabag goes in when the water is not boiling or it will taste bad) and coffee. spicy foods also have that effect, if you like them (personally i'm in love with them but i know many people can't stomach them!).
5) decreasing the mass of food you take in will result in reduced "water weight" or "water retention." this is the mass of food and water in your digestive tract, adding weight to your weigh-ins. don't panic if you've eaten a lot and see a kilo's gain! no one can gain a kilo of fat overnight, unless they've eaten thousands and thousands of calories---this is simply the weight of the food you haven't evacuated yet. this is especially true if you've had a lot of sodium-heavy (salty) foods, aren't getting enough fibre, or drink too little water. more on that later.
6) all calories are not created equal. some people will tell you "a calorie is a calorie" and "weight loss is just CICO (calories in, calories out)" but unfortunately, that isn't true. fats, carbs, and protein all take energy to digest: that's called the thermic effect of food. this is measured as a percentage: the calories burnt by digestion relative to the calories supplied by the different categories. generally, fats have a thermic effect of 3%, meaning that consuming 100 calories of pure fat would burn, through digestion, 3 calories. carbs (fibre included) have 5-10%, and protein, 25-30%. this is why high-protein dieting is such a staple of weight loss---there really isn't a disadvantage to upping your protein intake.
7) while we're on the subject of protein... it helps keep and build muscle through the weight loss process, which is great, because a calorie deficit will burn muscle as well as fat. now, muscle weighs more than fat. this means that, at the same weight, a more muscular person will have a much slimmer, not to mention more defined, figure. an easy way to increase protein is, of course, to eat lean meat, but also to include protein bars!! personally i like the brands Barebells and Fulfil.
8) and since we're talking about macronutrients, i'd like to mention fibre! while it (being included in carbs) has a low thermal effect, it presents the advantage of being non-digestible. that means it goes straight through the digestive tract and cleanses it, making you, in crude terms, ****. "who cares about that?" you ask. well, it reduces bloating and the aforementioned water weight---not only does your stomach look flatter, but you also get a more accurate number on the scale!
9) excessive fats of any kind (milk/dairy, oils, anything) make your skin and hair oily. it seems obvious, doesn't it? it's not related to weight loss, but it's really quite a convincing reason to cut back on them a little, at least imo.
10) highly processed foods are stuffed with extra sugars that make them extremely addictive---companies do that on purpose to sell more. glucose sends an easy shot of dopamine to the brain, which is why people with ADHD, depression, and other conditions that stem from a dopamine deficiency are likely to get addicted to sugary foods. if you find yourself eating lots of sugary things without experiencing hunger, this is probably where it comes from. try finding different sources of dopamine, like stroking a purring cat, listening to music, playing video games, finding and playing a sport you enjoy, having an active social life (for extroverted people)... and keep sugar-heavy processed foods out of your house!!
11) despite the misleading name, drinking water actually helps to eliminate water weight in much the same way as fibre, as it is flushed out quickly. on top of that, it keeps you full, holds cravings at bay, and helps with clearer skin! what is there not to love? to figure out how much you should be drinking, try to divide your weight in pounds by 2 and drink that amount in ounces. don't drink too much though, you could overload your kidneys and end up with kidney stones. (this is also why high sodium intake increases water weight---it soaks up the water which would make us un-bloat.)
12) diet pills often just help you flush water weight. most of them don't make you lose real weight at all and can be dangerous!! i'd suggest you do some in-depth research on the functioning of any brand of pills you might consider using. many of them are simply disguised laxatives or diuretics.
that's all for the physical side of it! of course, there's a lot of mental work to do, especially for those who comfort eat, rewarded themselves with food, struggle with binge eating disorders... will for self-improvement in all areas is usually the most effective motivator, not just dissatisfaction with your body. hope i could help <3 good luck and take care of yourselves!!