If you see the job advertisements and phone numbers posted outside the store. So should I call or just go to the store and ask?

Go straight in and tell your boss that you want to apply. The phone call just gives you time to think about it. You call and say you want an interview, and then you still have to face-to-face with your boss, or you can go straight in for an interview. Do you want to leave a phone call after the interview? So don't think too much, but think about whether you can do an interview well, seize the opportunity and wish the landlord success.