Okay, first of all, you are going to fail because you are not doing this at your own volition, but rather, because you feel forced to do so; even the clinic won't help you as it will be more of what I said above.
Read the sticky, to start with.
Disregard cardio, fast, extreme diets, and such ; you don't need to torture yourself to avoid a punishment, take this as an oportunity to discover a road of self-improvement that will make your life better in every sense, that might be even get you some pussy, you virgin fatso. Even if you don't lose as much weight as fast as you need, I think that as long as your parents see that you are doing an effort they will be happy, but you actually have to put it the effort.
Don't do this because you want to avoid the clinic, do this because you actually need to improve yourself. And there is nothing better for be willing to lift than actually loving to lift.
Start an aestethics routine, not cardio, just lifting, like Starting Aestethics or one of that meme routines, something simple that you can keep on. Also, download habitbull and set up a goal for yourself like giving up on sodas/shit food at least 6 days a week, mark in the app with red every day you fail, and with blue every day you succeed: your goal now is not to lose 20kg, but rather, to fill your month with blue dots.
Download my fitness pal, count calories, eat tons of protein, make yourself some tasty meals (its not about torturing yourself) but avoid fast food, stick with it for two weeks, weight yourself. If you have lost weight, mantain the daily calorie goal (there are online calculators for this), if you haven't, lower it (-500), if you gained weight, lower it quite a lot (-750). Repeat until you get a more accurate idea of what your actual tdee is.
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