Any good websites that teach you how to get better at doing math in my head?

[mii]

I am usually great in math. I always get As but i its hard for me to do it in my head! even simple problems. I always have to write it down. This is a problem i have secretly lived in since i learned math. lol does anybody know any site that will help me get better at doing it in my head because my mother need me to be "cashier" at this thing her and her friends are organizing and they wont have a register for me to do it on. im freaking out