First things first: it doesn't matter where you work, you're making money. People who criticize fast food employees are either people who have everything handed to them or have never had to work in their lives.
As 16YrsOld said, a buttoned up shirt and some pants (maybe some dark colored jeans) would be excellent. It'll show you're wanting to take this job seriously, even though you'll never be wearing a buttoned up shirt at McDonald's. Employers are looking for seriousness, not laid back and "chill" workers.
Some companies are just as informal as what you mentioned about your dad. For example, I work at Advanced Auto Parts and was called in for an "interview". Guess what? I was never interviewed, I was TOLD that I had a job and never had to go through an interview.
The interview won't really have you doing anything. From my experience, my interviews have all been questions; ranging from possible work hours to if I've ever had any experience with a cash register. Cash registers really aren't that difficult to manage so don't be scared of them.
Be confident in yourself. Don't go there thinking you're not going to get the job but also don't go in there with the 100% security that you'll get the job, it's happened to me countless times before that I think I nailed an interview and I never get a call back (that's just how some managers work). Don't get agitated over not receiving a call from them either, there's loads of job opportunities and you should always keep your chin up.
Good luck, Maj. :*