The day has come to have your trial and just like you wouldn't turn up to an exam without studying, or to a work presentation without preparing, it is an absolute must that you do your research before coming to your artist!
This means bringing tear sheets from magazines or images from pinterest/google/instagram of hair and makeup looks that you like. Never mind if it doesn't seem realistic, your artist will be able to assess the images and advise you what will and won't work. It is hugely helpful have these images as it gives the artist an idea of your tastes and general style. That way they can tailor a look that is perfectly matched to you.
Have a friend come along. It's always good to have a second opinion, and they can be on hand to take lots of pics for your own reference!
Take care of your skin by drinking lots of water prior to the trial. Also prepping your skin by cleansing, exfoliating and moisturising will help the products glide on.
Come with clean and dry hair (washing the night before is perfect). Your artist can easily dirty the hair up if need be with product, but it's a lot harder to get rid of oily tresses.
I hope this was helpful on your journey to your big day!