If the supermarket uses a standard layout, there will be an area to clock in, an HR office, and a break room. If you don't already have your uniform and badge/ID, you should plan to arrive early and head straight to the HR office to finish onboarding. If you already have those things, clock in with whatever their process is, then look for either the department manager you are working under or the HR manager if you don't know who that is. From there, your DM will handle getting you familiar with the workplace, the processes you'll be responsible for, and the team you'll be working with.
General advice for starting any new job: the manager knows you won't know what you're doing, and will usually keep an eye on you for a while to be sure you can handle not getting someone hurt or breaking anything, don't feel awkward if you feel like you're constantly being watched on your first day or two. The manager also knows that they will need to get you up to speed, so just follow along with them. After they get ready to let you get to work, be sure to ask the most important question: "Who do I go to if I need help?" A lot of managers in supermarkets designate a specific team member as the person for general questions or advice, and they don't want to get involved unless the problem is big, so be sure you know who to get with if you need more work or have a small question.