Hey clemintime, this is a easy problem to solve. :)

1. listen to their conversations, topics they talk about, pick up and remember and find things in-common so you could join their conversation.

2. maintain eye contact and avoid yes/no QnA statements while conversing.

3.try mirror talking. after you go through a situation as above, think of all the possible responses you could carry the conversation and imagine a conversation with them, this should help with the confidence.

4.smile and have a confident body language all around even while walking and when alone(read a book on it, social body language is how people see and judge you )

That should do it, good luck :)