I've always struggled until recently. I always smile (I think it reaches the eyes!) I always saying good morning! Ask how people are. Offer up a drink and some small talk.
And I always talk to people about things that I think will make them happy, usually holidays, their kids or their dogs!
For teens, it'll be things like their clothes, the music they like or their hobbies. If DD has a habit of over sharing, tell her to just take an interest in people, and take their lead. If they share how their weekend was, she can reciprocate, if people talk about their health, then reciprocate in the same sort of manner.
I never thought I was great with people, but DH told my boss before he met me "alligator gets on everyone's good side. She finds out people's life stories the first time she meets them!"
They both call me the chameleon now, and I get sent in as the fixer of fractured relationships.
Just being nice, being perceptive to what people need and figuring out peoples likes and dislikes has turned things around for me.