I did my first tri last September though we had some practice doing transitions. Nothing prepares you for the real thing though! I put on bike shorts and jersey over swimsuit and then ran in them with a hat at the end. Now when we practice transitions i run with sports bra over swimsuit (hey, some girls are bigger than others! ) and that works great ( only you may have to do the "Bra Dance" to get it on with wet body, but enough practice and youll be good!)

Dont worry too much about it, in my race there were some people out there wearing some VERY interesting clothes- inevitably some old guy wears a Speedo or a flaming yellow pair of really small running shorts he shouldnt be wearing. And who cares what you are wearing, so long as you are out there and doing it and having fun!