How do I get a part time job that's only a few days per week?

I struggle with anxiety and OCD so I want to get a part time job that's around 3 days per week and like 20-25 hours. I've never had a job so I just want to get myself used to working and also have a bit of money saved up so I can start working on getting my license.

Most part time jobs I see don't mention hours or anything, and I read online part time is usually like 30 hours. Am I supposed to tell them how many hours I want to work when I apply or am I supposed to find a job that specifically mentions 3 days/20 hours a week?