You work the cash register exactly the way they will teach you to work it. They prefer to train you themselves on any equipment you will be using. It's perfectly ok not to know how to do it when you apply for the job. Cooking the burgers is easy there, too.
Because they plan to have such a high turnover rate (most of the non-management jobs at McDonald's are considered "starter jobs" where they expect you will work there for a year or two and then move on to something else) all the jobs at McDonald's are designed to be easy. Anyone who listens well to their training can do them.
I worked there for a year, the register is easy. Before they even put you out on the floor you will have to watch about 3 hours of videos on telling you what all to do and how to do it. The register has picture on it of the food, i mean its really simple. i could do it in my sleep lol. and where you make the food, there should be pictures hangin up in front of you on how things go. But basically I can tell you that if you are a girl, you wont have to worry about making the food at all except for the fries. you will only do frontline or presenting, presenting is when you hand food out the window, frontline is when your on the cash register. you will do just fine.