Introvert who wants to talk to people regularly

I'm a stammering introvert and I don't feel like talking to people that when buying things I'll just gesture for the price, pay with a card and be on my way. However, sometimes when I see friends or colleagues chatting happily, I want to join in the conversation but chicken out due to not believing I can contribute anything and scared to talk. I'll even take weeks to text my friends unless they chat to me first. Anyone got advice on how to deal with this contradiction?