This post is about starting your first day in a restaurant. Hopefully, they have told you what to wear or have provided you with your uniform. If they haven't provided you with a uniform then it may be the first thing they do. If they don't tell you what to wear and don't provide you with a uniform, use common sense and wear clothing similar to everyone else's. You may not have a Sonic uniform, but a red polo and black pants are easy to come by.
If you haven't already you will sit down and fill out your I-9 and W-2 information. Be sure to bring your driver's license. social security card. and a pen, if you haven't done this yet. You may also spend some time reviewing a policy manual/company handbook.
It's very common for a manager to pair you up with a seasoned employee. This style of training is called the "magic apron", because the manager assigns you to an "apron", another employee, and "magically" you learn how to do your job. Occassionally, some places will have you read through training material, or have you watch a short video or series of videos, but that is uncommon. You'll be more likely to see it in a restaurant chain, like McDonald's or Burger King.
Occassionally, the restaurant is in such dire situations that you are thrown in to learn as you go, so it's probably a good idea to be familiar with the menu before you start. Remember to stay calm and ask a lot of questions. You should ask about the menu. Examples: Is there anything on the menu we don't have?, Can I substitute things?, What is the vegetable/soup of the day?. Spend any down time you have looking over the store to figure out where things are supposed to go. For brownie points with management take a towel and wipe down things as you look at them. This will help you remember where things are. Physically touching an item and saying to yourself "I wiped down the credit card machine" will allow you to really remember where it is.
If you've never worked in a restaurant before, you'll soon discover how fast paced and busy it can get followed by breaks with no customers in the store. It's important to stay focused when you are busy and keep moving when you are slow. Nothing will make manager's or owner's more upset if they see an employee standing around doing nothing. "If you've got time to lean, you've got time to clean," is a common phrase you'll probably hear.
On your first day you should try to absorb everything like a sponge. Make sure you really focus on your job. If you're hired as a cook don't worry about the expo guy, just being a cook. It's best to learn the proper way to do any particular task because most employee's will have developed shortcuts they may want to teach you. These may be inaccurate so learn to do the job right.