Hi Jessica,
I know full well what you're worried about, and it's a tough decision when you don't want to reveal this side of your life, yourself. For what it's worth, and coming from someone a bit long in the tooth and completely closeted (sadly due to my career), I'd kill to go back to when I was your age and have this opportunity. OMG...To have a young woman help you dress up and to go out with you? Such things don't happen for many of us, certainly not me.
How should I act, both towards the dressing up and once as a girl?
Just be yourself, saying you took a dare. Ask laughingly, "So, how do I look?"
What kind of outfit would be best for this occasion? Are there parts of women's clothing I definitely should(n't) wear?
I say go as beautiful and/or sexy as you want. Really, once you jump over the gender fence, you're in that yard and the degree to which you're there really won't matter. Allow your inner dreams to come true while, if needed, presenting yourself as it all just being a crazy, fun lark.
Hope you do this, but only if you promise to have fun!!!
