Be genuinely interested in people.
I remember at secondary school I had a friend who used to remember every time someone went to the dentist, their granny visited, their dog was unwell etc and always asked after them in a way that felt concerned, not intrusive.
So remembering things that matter to others, and ask how things went. But not continuing asking so it feels intrusive.
It's also practice. My siblings struggled socially. So they chose not to do things where they might have to be sociable, so they didn't learn. It was a circle. They also tended to blame others (not to them, but when we were home) rather than feel they should make any effort.
I am not brilliant socially, but they would say (possibly through gritted teeth ) that I am very sociable and it's not fair . I would say I learnt a lot from them what not to do and how to react.
So giving it a go. Remembering what works and what doesn't work.
Then there's making the most of what you do know/have. We didn't watch TV much. My siblings would say how terrible this was because they couldn't talk with the others at school. I had a known niche as the person people could come and enthuse about the latest TV programme. My siblings would stop the conversation with "I didn't watch it." I'd listen and make comments/questions, which helped when making friends.
So don't stop conversation when it's happening. If it's not interesting to you, then listen. Don't give people the impression that you only want to talk when it's about you, or something you do.
And doing something together can be an easy way of making friends. A joint work. Often things that seem boring can really bring people together. I played tennis, and we wouldn't have been friends in the normal way, but playing on a team meant we spent time together and had a good laugh. Equally well my very shy dd finds working backstage on performances has brought her best friends. Doing something together, working towards a common goal really brings people together.
So do some activities that you see the same people, but with something that you are working towards together. Pick what you do, but don't go in, seeing the people as first and foremost potential friends, see them as people first.
Don't make snap judgements. I've got a couple of good friends that are not the "normal" people I would hang round with. The person who let you down the first week, may normally be very reliable. The person that told you they can't stand your favourite musical, may share other tastes with you.
So give people a chance. Don't write them off the second they do something you're not sure about, or because they don't look like your sort.
From one not-very sociable person to another. It's hard. I find it exhausting. But it's also rewarding. It takes time to build up a friendship too. Don't expect it to happen at once.