I don't have any pics of me when I was super skinny. The reason is basically because I was absolutely, 100%, totally ashamed of how I looked. Any pics of me from that era show me wearing oversized clothes that hid my body. The worst part of the oversized clothes was that they were actually the smallest adult sized clothes that I could ever find. Everyone would tell me to buy boys' clothes (how embarrassing is that?), but those didn't fit me in the crotch (God's only break for me). The most difficult thing for me was to overcome my shame enough to hit the gym.
I was fortunate enough in college for a big buddy to practically drag me to the gym because he didn't want to work out alone (I'm guessing), and he had seen me in the public showers and told me that a guy with my endowment deserves a better body. Whatever works. But the point was that once I overcame my initial shame, I knew that I would be alright.
As
@FootballBro25 pointed out, it's a combination of "diet & exercise" as the cliche goes. There is no magic pill. You can't get to a point as stop. You have to keep at it for the rest of your life. Once you reach adulthood, your body begins to deteriorate at an accelerating speed. I like the metaphor that your body is like the only car that you will ever have, and that you cannot ever trade it in for a newer model. Therefore you must treat it with kid gloves.
Probably the biggest hurdles for me, once I was able to get started, was knowing how to eat and how to train. I used to be uninterested in eating (unlike 99.99% of the human population). And once I started working out, I found myself hungry all the time, but never getting enough of the protein I needed. Then there was the problem of not knowing how to train properly. I seemed to always do too much weight and never getting to the muscle burn stage because I'd tire out beforehand.
But going in my favor is that I really like to lift. I enjoy it because I've never felt stronger or more alive that I do when I get my pump going. Of course there's the benefit that a lot of people now want to have sex with me, but the sex itself is not the primary motivator for me.