My first nudist experience came when I was 17 (I'm 24 now for reference). I made friends with a girl in high school that year named Veronica, and I later came to find out that she and her mother were both nudists.
A few weeks after that, I was invited to dinner at Veronica's house with her and her mother. Veronica's mother wasn't home when we got there so she invited me to go swimming with her in their pool. Veronica of course stripped down naked and dove in, but I was a bit more apprehensive. I hadn't been raised with the same kind of acceptance towards nudity that she had, and as a result I was a bit uncomfortable being naked in front of anyone, not to mention I was very embarrassed about the size of my, well...you know.
Anyway, I elected to go swimming in my tightey-whiteys since I didn't have my swimsuit on me, and we had a great time in the water together. At some point my underwear slipped off in the water and I didn't even realize it. After getting out of the water, I heard Veronica giggle and she said to me that I looked good naked. Of course I immediately turned red and covered myself up, but after she pointed out that she had already seen me naked and there wasn't much use trying to hide it anymore, I relented, and despite my previous worries jumped back into the pool naked.
Much to my surprise I loved how much fun and free swimming naked was, and telling this to Veronica she responded by saying that's how she felt everyday and that's why she loved being a nudist so much. Not long after that Veronica's mother returned home and was very surprised to see me naked, to which I joked that her daughter had converted me. I decided to stay nude for the rest of the day, had dinner with them naked, and that experience ultimately opened my eyes to what a wonderful lifestyle nudism is.
Once I went to college and moved out on my own, along with a lot of encouragement from Veronica, I decided to become a nudist full time like her and I've been one ever since. Veronica and I also started dating that year and we're still together.