I think being aware that you need to think outside of your regular social circle is a good start. But there are plenty of people you encounter on a daily basis who may not be in your social circle.
I know of many children/teens who are simply incapable of interacting with a “stranger” in any context.
A small, very easy and achievable way to start modelling more confident behaviour is to make friendly small talk when going about your daily routines.
Smile. Make eye contact. Don’t complain. Don’t swear. Say something nice or at the very least don’t say something nasty. Know when to stop.
If you can’t say hello, you’ll hardly be able to make conversation at the local whatever group.
(I’ll put my hard hat on and say that many adults in my community on both ends of the income scale are not capable of this either. I’ve noticed that English Home Counties folk are extraordinarily insular, no matter if they have money or not)
On another tangent, in my opinion, appearances also matter (unfortunately) so fostering a basic self-care routine: being hygienic, brushing your hair, wearing clean clothes. This all contributes to being more confident.