Look at it this way on your conquest to become "fit"....
The body is more intelligent and adaptive than what most acknowledge. It will adapt and overcome adversity and become accustomed to set routines. In order to further yourself along with overall development, you must continually manipulate and alter the variables that you utilize in your exercise regime. The plateau is a real thing, but it is easier to avoid it with proper knowledge than to get stuck on it and fight against the relentless tide.
Now in regards to becoming fit within the smallest period of time...
Learn proper nutrition. Most humans teeter on the minimalist approach to their nutrition. Some may adequately cover most areas, but in order to achieve maximal results you have to become superior in your nutrition. When you first go about crafting a rough draft for a diet to follow just keep it simple. There is no need to over complicate the process and become frustrated like so many before you.
Just follow the simple yet most important rule: If you want to gain weight you have to consume more calories than you expend (hypercaloric state). If you want to lose weight you have to burn more calories than you consume. (hypocaloric state)
Personally, I was a skinny little kid all my life and always passed it off to my genetics. I tried everything in my teens and even used AAS before I even knew what I was doing. I never got the satisfactory progress that I envisioned for myself until I sat down and took the time to educate myself on nutrition. You will learn there is no black and white with dieting. Honestly it is just gray 24/7. There is no perfect guideline and even after you understand your body's individual response to the things you do and become in tune, it will still need more fine tuning.
It is a arduous journey regardless of your genetics, willpower or discipline. The individuals that are successful are the ones that are always willing to learn. The guys that funnel hedonistic broscience into your cranium are the ones that never got the memo.
Good luck! This board will help you with anything. just ask.