what @alxndrhll said.
When it all boils down, weight loss is simple maths - put less fuel in than you burn and you'll lose weight. It can be whatever kind of vegan/keto/fast food diet you like but as long as you're burning more than you put in you'll use those fat reserves that we all want to get rid of. It's the focus on calories and the knowledge that it gives you that'll help loads, when I first started using an app to track them I was amazed at exactly how many calories a large bowl of cereal could have, at how easy it is to have used up a days calories by lunchtime and also how portion control is a really useful thing to keep an eye on too.
Biggest thing is willpower though, it'll be tested and tested again both in terms of food (we're all guilty of late-night nibbles!) and also any kind of exercise you do, the hardest part of exercise is always getting out the door / starting - you feel great after but sometimes when you ache or it's raining or there's something good on the tv it's far easier to not do it. Training buddies help a lot so why not arrange something with a mate and have a catch up over a ketlebell or run rather than a pint? or have the pint afterwards, you gotta rehydrate after all 😉
Get some scales, but don't use them too often and don't get down when they go the wrong way. Weighing every day does no good at all, weigh yourself on the same day each week at the same time. General consensus is first thing in the morning after you've been to the loo, but the same time (whenever that is) helps to reduce the variables. The scales will go the wrong way, don't ignore them after that and don't throw them out the window, it's just a reset - ok, what did I do this week that's caused that? what am I doing differently this week? Losing a load of weight can be daunting, so those weekly targets can be good as stepping stones along the way, losing 20kg is a big number but making that 200g a week, well that sounds more achievable.
About 200 calories a pint. Just factor them into a weekly meal plan, or do some extra exercise to balance it out. For me, a pint is about 15mins of cardio.