Some time around age 15 or so, I decided that that wasn't how I wanted to live life, so I worked on starting to change. I started looking at myself and my own behaviors, listening to myself, and watching and listening to other people and how they interacted. I started reading self-help books like 'How to Win Friends and Influence People' (deservedly a classic, and one of my favorite books to this day). I worked on spending more time with the few classmates I could count as friends, and made more of an effort to become friendly with people I didn't know yet.
Books can help point you in the right direction, but they can be an escape if you're not careful. What helped in the long run was putting myself into different kinds of groups of different kinds of people and challenging myself to make the best of it.
It was a long, slow process, and 40 years later it's still a process. It was a lot of hard work. There was unlearning as well as learning involved. But now as a 55+ adult, I can honestly say that - even though I'll always be an introvert by nature - I'm comfortable around people.
Staying at the hostel the past few days brought this home to me. I thought I'd feel uncomfortable and intimidated among a lot of strangers - many of them from very different backgrounds - but I don't. I like people. I don't like everybody, but I like people.