Advice for getting into shape

Some general truths:

• Drink a lot of water. Fat requires water in order to be converted into energy. If you don't drink enough water for your energy needs, your body will burn muscle instead of fat (which seems counterproductive to your goals).
• Calories in, calories out. If you're looking to maintain or gain weight but shift how much of it is muscle (makes the most sense, tbh), then you will need to be eating enough for your body to build muscle without burning as much muscle as you built. So eat enough and make sure enough of what you eat is protein (though the research on how much protein you should be eating is pretty spotty: just aim for "a bunch" and you'll probably be fine.)
• Weight training is the best way to build muscle. Cardio is the best way to get used to using muscle. You should be doing a bit of both any given week, but make sure you hit all the main muscle areas when you train. "Never skip leg day" exists as a mantra for some people for this reason.

My advice for maintaining basic fitness even when you're old and wrinkled and grey is to get into positive habits. Take the stairs instead of the elevator. Consider doing most of your trips in a ~10 mile radius on a bicycle or on your feet instead of a car or public transit. If you live in a mountain range, or a place like San Francisco, an e-Bike is a good option. You still have to move the pedals yourself, the motor just helps when you need it.
If you're sitting down a lot at home, consider a standing desk. Or a desk that can switch between sitting and standing height. You're less likely to move when you're sitting down.

Count housework and yardwork as exercise. You'll have a clean house, and you'll have burnt some calories.

If exercising is just part of your daily life and serves a purpose, it's easier to maintain.

Burning fat happens fairly slowly for most people. It depends on a lot of different factors, some of which I have experience with and understand decently, some of which are totally beyond me. I'll try and do a decent job of explaining the few bits I think I understand.

Like @BuckeyeSundae said, calories in/calories out is generally a good starting point. Figure out what your basal metabolism, or BMR is. There are several BMR calculators, don't expect them to be accurate, but you're really just looking for a rough idea. Any work you do in a day will add to that number. Take any calories you consume and subtract from that number. If the number you're left with at the end of a day if net positive, your body is going to have to make the difference by burning stored energy.

Things get tricky about how your body reacts to its environment to decide where exactly it gets its energy from. Most of the time people want to lose fat (I honestly can't think of a reason why someone would want to "just be able to get rid of all this shitty muscle mass", except for maybe actors prepping for a role). But the crappy thing is, most of the time our bodies don't want to use that fat. It's kinda there in case of emergencies. If you're able to meet some criteria, your body will shift into the mode to burn fat, or "ketosis".

If the ratio is too far out of whack, you get undesirable results. Consuming too few calories can cause a whole litany of bodily issues, especially over a long period of time. What I've seen from studies I've looked through in the past, common thought on the subject, and personal experience is that typically a 500 calorie deficit is a solid goal. This results in burning through approximately 1 pound of fat per week. That might not seem like much if you're trying to drop a bunch of weight, but adopting a long term discipline/regimen is usually requisite.

On the bright side, even though a pound a week may not seem like much at first, it can make a fairly big difference in a relatively small amount of time. A pound of fat can be a pretty surprising volume. Check out "pound of fat vs pound of muscle" for a visual illustration. And say for instance you only go for 6 weeks, and only burn through 6 pounds of fat. While it might not seem like much when you step on the scale, your organs will probably think it's a big deal. Visceral fat is the stuff your body packs away inside, crammed in around your guts. The stuff wreaks havoc on overall health. Getting rid of it might not make a huge difference appearance-wise, but you'll feel it in your daily energy level.

I can go on a bit more if any of this is helping.

Cut out soda. You could still be a couch potato but cutting out soda will still make a massive difference

Eat healthy, maybe join a sports team at your school (or just a local club.) That will net you some friends, and making exercise both scheduled and fun is a great source of motivation.

I recently got a 3 months gym membership for a similar purpose. Wish you luck! This has been the best resource I've found so far.

Exercise bike is good or a real bike but that’s dangerous. So two of the radio show/podcasts I’ve listen to did prolonged weight loss challenges. Someone shrugging with weight ate boiled unseasoned potatoes, apparently that helps. Also for weight loss, look into this stuff called Swiss Kriss. It’ll make you shit a lot at night, but heart health is worth it.

I'll toss a few tips out there. Losing weight is way more about diet than exercise (though exercise is important). Don't drink your calories. Sugary drinks are high in calories and not filling. It's way easier to stay active if you do it socially (sports, workout buddy). I highly recommend stretching and improving flexibility. It will prevent injury.

I'm going to start with this tomorrow. https://www.reddit.com/r/bodyweightfitness/wiki/kb/recommended_routine