Thats might actually be good for you for the first time, becasue it may eleviate a lot of pressure. My first outing was similar, me and a friend went to a haloween party that was mostly Gay and otherwise gender variant, a lot of people their new me, so I did not feel that nervous. I mean honestly in my mind, I did want to pass for female and I was trying to look my best, but at the same time, it was a load off to know that just in case people their knew me and it was a friendly crowd so I could relax and have fun. We went as ken and barbie, we made the boxes and everything and just had our hands and feet hanging out, it was really fun.
But genearllly though like in other everyday places, I just obsreved what other girls around me were wearing that were my age and just tried to blend in the best I could. I've always been a bit self cinscience anyway, but I never wanted to stand out that much, just look ordinary. So just find a style that you are comfortable with, and one that matches your personality. As you transition, you will find a style that suits you.
Have fun on your nite out and be safe
-pass-