Yeah you'll be fine don't worry. Cashier is an incredibly easy job; when I was a freshman in college, I worked as a cashier and stockboy at my local market, and it wasn't bad.
One thing I'll tell you though is this: do not take your job very seriously.
While blunt force and insane work ethic might function for good grades, they will actually hurt you in the working world.
Your boss isn't an objective machine, your success is determined by whether or not he / she likes you or not. If you annoy your boss by being an overachiever, they will punish you directly and indirectly.
I was an A-type person at my first job, and my boss made me do all the shitty janitorial work that nobody else wanted to do - it was disgusting - and he would threaten to fire me for the most trivial shit (like him posting the week's schedule after my shift and blaming me for missing a shift).
Become friends with your manager as time goes on, and get a feel for the work ethic of the place you're at. If everyone there smokes weed in the bathroom, you shouldn't act like a bomb-diffusor in your work.