Well done you for booking it.
Basically, the nurse will get you to lie on your back, and will probably get you to put your feet together and spread. Your knees. She'll use a speculum to open your vagina. A speculum is smooth, clear plastic, and it opens (gently) outward. They used to be metal and quite cold! She'll probably smear it with ky jelly first. Then she takes a brush-like thing, about the size of an earbud, but longer, and rub the top of the inside of your vagina (cervix) to collect cells for testing.
It's not wonderful, but it's not horrendous either, let her know it's your first one and that you'd like her to explain what she's doing. It's over really quickly too, so just do it and get on with your life! It's a whole lot less scary than finding out much too late that there's something wrong. And she's done it a million times, so you'll not have anything she hasn't seen before.
x good luck!