Hi!
Unfortunately an interviewer is often as interested in how you are as a person as they are in your answers to their questions! I totally understand your shyness worries as I was so shy when I had my first job interview I thought they would never hire me - luckily they did! My top tips if you are shy are to make a conscious effort to smile (if you are nervous your face can start looking worried without you realising it), speak consciously a bit louder than your instinct tells you to (similarly when you are nervous you can start to speak quietly without realising it), and just come across as generally friendly (though this isn't as much of a problem as friendliness generally comes across even if you are shy).
In terms of questions, they will probably ask you:
- Customer service based questions - what do you think makes good customer service? How would you deal with a customer complaint?
- Scenario based questions - what would you do if...? You often can't prepare for these but if you don't know be honest and say that you would ask somebody what the policy was so you would know next time.
- Experience/suitability questions - why do you think you are suitable for the job? Have you had any other experience or training to do with food or customer service? Have you had previous activities where you have demonstrated teamwork, dealing with the public, reacting to situations, dealing with money, etc?
- Practical questions such as Availability - what days and times can you work, and Transport - how are you going to get to and from early shifts or late shifts?
To be honest for a lot of jobs at that level, once they invite you into interview they basically want to hire you off the basis of your application form but want to check that you are a normal, friendly approachable person who can work in a team and be professional.
So don't worry, smile, be honest rather than freaking out if you don't know what to say, ask the interviewer a question (if its a new McDonald's how long has it been open? What are the opening hours? How do they decide who cooks and who serves? What training opportunities are there?) and you will be fine! Good luck!