When I was in middle school, I was super quiet. Like, I would go entire lunch periods without saying a single word. I just didn't realize that I might want to or be able to actually interact with other people. I had a cognitive impairment which stopped me from expressing myself, but I didn't realize it at the time. I eventually realized in 8th grade that I could not hold an actual one-on-one conversation, and I decided that needed to change.
So I had to start by learning the basic conversation skills and such, and eventually moved to actually making jokes and stuff mid high-school. By the start of college, I could participate socially as a mostly normal person. I just caught up on all the social skills I had to re-develop, like making plans and stuff.
All this seems really simple, but it is hard to catch up when everyone else is already friends with each other. Now, I might be one of the most extroverted people I know, and I almost always talk to people whenever I get a chance. I accomplished my life goal when I was 20, so I am really happy about that and I've been riding that high ever since.