First of all, don't worry. I know that's easier said than done, but my job is to guide you, and give you the confidence to forget I'm even there with my camera.

I'd recommend wearing earthy, neutral solids. This sounds obvious, but don't wear something that you feel uncomfortable in. If you're feeling uncomfortable, it'll show, and make it harder to move freely for prompts. Last but not least, don't take yourself too seriously. Sometimes the sillier the pose, the cuter the photo turns out.