I really wish I knew. I'm in my early twenties and in the same boat as you. I just can't figure it out.
I have zero friends and barely any acquaintances. I still live in my home town, but I had zero friends in high school, so it's not like I have any high school friends in town to hang out with. And I went to college in a different state. I barely made any friends/acquaintances there anyways, but now that I'm graduated, they're in different states anyhow.
I work at a place with mostly young people around my age, but I haven't been making friends there. I have been there for a year now and I talk with people in work, but nothing has grown into a "friendship" where we would hang out outside of work.
I feel like I'm in such a rut. Part of me is just figuring I should start feeling content with my solitary life, but the other part of me is still wanting friends and people to do fun things with. For example: I am going on a cruise with my family in october. It will be my mom and dad in one cabin, my grandmother and grandfather in another cabin/room, and then me by myself in a two person cabin/room because I have no friend to invite! Like, if I was "normal" I'd be able to invite one of my friends to come on this cruise with me, but since I have zero friends I have no one to even ask. It's more expensive too, to have just one person in a 2 person cabin. I will just be stuck with my parents and grandparents the whole vacation, but I do think I will still have fun. I just wish I actually had a friend I could bring
But I just went off on a tangent. Basically, I don't understand how we, as adults, can make new friends. I suppose just keep joining clubs and putting yourself out there and hope for the best? I don't know.
One tip I will offer though is to check out the website meetup.com. There are groups on there which do fun activities and most of the people who go to these meetings go by themselves because they are wanting to meet new people, same as you. So I guess a friendship might be able to grow out of that. I went to two meetups. One was a social anxiety group (it was kind of nice and helpful, but the people were actually older than me, while I was hoping they'd be my age). And I went to another outdoor adventure meetup where we went paintballing. It was very fun! I didn't really form a friendship with anyone, but I did talk to people and it was still nice to be able to have people to do something fun with.