Ask your parents for permission first.
You're their responsibility and they choose when you can do things on your own in order to look after you.
There's no minimum age for going to the shops. But your parents might not want you to go on your own until you're older.
None of us can tell you how old you should be, only your parents.
It depends on where you live, how far away the shop is, what the neighbourhood and roads are like, what you're going to buy, and whether your parents can trust you to be safe, behave, and go straight to the shop and come back.
Ask your parents. Do not go out without their permission.