I thought the initial ones felt a bit like gas bubbles moving around but ever so slightly different, if I was paying attention. It then progressed within a week or two to the sensation one gets when a muscle twitches involuntarily. It's still a bit like that but much more defined as 'kicks' and 'rolls' as she's bigger and stronger now.
I started feeling her around maybe 16 weeks. However, I have a posterior placenta. If yours is on the front, it'll be a lot harder to feel those first movements.