Dressing: No jeans. It may be fast-food, but at least wear khakis or some dressier pants unless this is a family-owned business and the interview is really casual. At least show them you care enough to clean up.
Prep: Go to their website and read all the facts on the company. Then, Google them and see if there’s any current news on them. Company specific questions may not come up, but you’ll know you’re stuff and feel more confident.
Kevbo is right. They aren’t going to expect you to be expectionally skilled right now, but what they will want to know is that you’re responsible, dependable, driven and care about the customers – all with some basic math skills. They will undoubtedly want to know “why do you want to work for McDonald’s?” Know the answer to that. Why do you want to work there?
Also, if you’ve done some basic things in the past, shoveled snow, had a paper-route, had a lemonade stand etc. Let them know that. It shows that you take initiative and have a good work ethic.