plenty of change! you don't want your first couple of customers to clean you out and then be stuck.
do you have a brand - even if it is just a simple logo you could stamp on the paper bags you'll put the items in.
make friends with another at all holder for when you need the loo.
price tags or price list.
attractive displays, different price ranges.
snacks and drinks - you may not get much time to eat, so foods which don't make a mess and are easy to eat.
maybe have a competition - you'll get lots of people's details.
introduce yourself to the organisers when you get there and tell them it's your first fair and is there anything you particularly need to know.
check what time you are allowed to set up and take down, and what time the fair runs from and too.
check whether you get a table and chair supplied.
look well groomed, but not so immaculate that you are off putting. remember to smile.
put on your Facebook page that you are attending the event, and maybe e-mail any previous clients to let them know where they can find you to buy your new items.
good luck