I've come across many people who don't drink coffee without milk because of the bitterness however most of those people had never had a proper espresso instead they got caught up in the trendy hype of coffee with pictures in the milk with weird and wonderful names, shapes and sizes. I've managed to convert a few milk only people after I'd made them a good old fashioned espresso. To answer your question, I think a short macchiatto with just a dash of milk might be a good way to ease into it but more importantly if you're not making it yourself, you'd need to find somewhere or someone who knows how to extract an espresso properly as I believe this drink is the hardest one to master as it cannot hide any defects behind the milk. Good luck, I'm sure you'll know what I mean once you taste that perfect cup of liquid gold.
PS,
Ask for it as espresso, short black doesn't do it justice.