You're right, I was going to say panties, and then I read the rules! lol ...
That makes it hard for me to pin down, but I could say an influential early experience from wearing girl's clothes was the first time I tried a one-piece bathing suit. It was awesome. I was about 14, and I was using my friends bathroom, which also had a section they used for laundry. I had already been wearing panties since I was 11, and my curiosity led me to peek in the laundry area. I saw his cute older sister's green and white bathing suit hanging in the laundry area. Since the bathroom was locked, I felt safe enough to undress and then slid the bathing suit on. It was so hard to keep my hands off myself wearing one, they are so soft and sexy to touch. I wish I could've worn it out of there.
This was 1984, and tolerance of gay or cross dressing behavior was not very well tolerated, and in school it was essentially social suicide among your peers. By this point I had already been riding my bicycle up to the local mall and buying panties whenever I had money, but I was always petrified browsing for clothes thinking I would run into a girl from school. Still, my next trip resulted in me buying a bathing suit of my own! The reason I think it was influential is that it lead to explore lots of body outfits. The 80s had a lot of lingerie options, and within a couple months of buying that swimsuit I ended up buying a corset, similar to the one in the linked below. This lead to another fun thing to wear back then, the aerobics workout gear. Talk about tight body suits that felt amazing!
First corset (looked similar to my memory)
https://s-media-cache-ak0.pinimg.com/736x/b5/aa/50/b5aa50f74c6ccef1473989e6ca11f4de.jpg
80's aerobics class!
https://img0.etsystatic.com/039/1/5444446/il_570xN.626127948_nh2u.jpg
https://s3-ap-southeast-2.amazonaws...ploads/2015/06/03100958/aerobics2-640x320.jpg