Start with following some person who posts about things you care. Sometimes respond if you think you can contribute to the topic, be nice and soon you'll get your circle.
sznowicki
joined 2 years ago
I go one level deeper. I make me friends use matrix. They can use my server or someone else but I want them to meet me there.
You can also use some tools for easier discovery of interesting people. Like my mastodon search. I sometimes use it and got few mutuals this way.
https://masto.kukei.eu/