How to request accommodations without getting turned down/fired?

I put in an application at Giant Eagle for my first job ever so I don't know much about this stuff. I have some physical disabilities that cause me pain, and if I see an opportunity where it would be appropriate, I would like to ask to be able to sit down (like if I'm at the cash register or something).

How would I go about asking for accomodations like that? Should I ask during the interview or after I get hired? What if they say no or think I'm lying about being disabled just to be able to sit down? Is that even an appropriate accommodation to ask for at a place like Giant Eagle?

Sorry if these questions seem dumb or silly. This is my first time doing any of this and I don't want to mess it up.