Don Gabor has some good books on this issue, but I don't know if they'd help you or not. They helped me.
First of all, don't expect to make intimate friendships through retail. For now, if you're really struggling, be happy to get a brief exchange of words (even happier if you initiated it). This seems to be a common pitfall for introverts--they underestimate shallow and frivolous interaction. When you get good at this,
then you can start thinking about the next step.
Are you mostly trying to make conversation with customers, or co-workers?