You'll be fine.
I do a fair bit of women's both OA and U18 and it's usually incredibly straightforward.
They'll give you an easy ride, generally Dispute no decisions at all and be most polite.
Banter is very different and it's usually pretty quiet compared to men's too.
What will probably surprise you more is the age spread.In U18 male most players are 17/18, where as with ladies the spread can be from 13-18, because of the lesser numbers of players they end up with less teams relating to specific ages.
That usually means they've played together for a few years.The older ones are slightly bigger and stronger physically, but more importantly braver so they tend to be the ones you have to watch for fouls etc.
What I would say is you couldn't have a better game as your first middle, for all the above reasons.
Most importantly turn up, feel confident and confidence will come. Act confident and they'll assume you are. Act nervous and they'll know you are.
Go out and do your thing. What's the worst that can happen? You get a few decisions wrong?
You wouldn't be the first or last ref to do that would you?
Good luck, let us know how it goes.