1. They're there to help you
2. They're not trying to trip you up
3. Be honest
4. Have fun
As Helen says, go dressed, it makes a far better impression.
Tell them of any plans you have to help your own transition, such as self-medication, changing your name, coming out at work/to friends, that kind of thing.
They'll understand that you won't have a clue what's going to happen, and they'll lead you through the assessment with gentle questions.
At the end of it, don't expect to be taken to a ward to surgically transition! Nothing changes after this, it is for them to gauge if you are a suitable candidate for their services. You wouldn't be going if you weren't!