The earliest I can remember was one day when I was about 13 when a shirt of my sister's got mixed up in the laundry and I decided to wear it to school, under a jacket. I remember loving how the shirt fit me but I didn't really think too much about it.
The first time I wore a full outfit was shortly after my 15th birthday, where one of my best friends (a girl) gave me a pack of panties as a gag gift. I actually liked the gift a lot more than I thought I would, and although I didn't want most of the people there to know, I wore a pair that night. The next day at school, I talked with her about it, and she was quite understanding, and actually a bit happy that her gag gift wound up being more than that. The day after that, she brought me a shirt, skirt, and some stockings to take home, which I put on as soon as I got home and spent the rest of the day wearing, locked up in my room.