My first full time job was as a checker in a grocery store. It's a really valuable experience for a young man, dealing with the public. You will learn a lot about human nature, especially when you are taking money from them and are expected to be at their service. It can get really scary and hair raising at times, and you will be frustrated and occasionally angry, but it will also renew your faith in people and their innate goodness.
It's something that will stay with you always, no matter what you actually choose as your career. I ended up using it as an means to an end; i.e. I worked part time there while going to college. Sharn Jean and I insisted that our daughters get similar retail experience, and it has served them well...they have required the same from our Grandkids.